Add apparent_repo_name macro to prepare for Bzlmod

Part of bazelbuild#1482.

Splits the last component off of canonical repo names to produce the
expected repo name.

Without Bzlmod, it returns the original name. With Bzlmod enabled, it
avoids generating output like:

    scala_import(
        name = "_main~scala_deps~io_bazel_rules_scala_scala_compiler",
        jars = ["scala-compiler-2.12.18.jar"],
    )

resulting in errors like:

```
ERROR: .../_main~_repo_rules~io_bazel_rules_scala/scala/BUILD:
no such target '@@_main~scala_deps~io_bazel_rules_scala_scala_compiler//:io_bazel_rules_scala_scala_compiler':
target 'io_bazel_rules_scala_scala_compiler' not declared in package ''
defined by .../_main~scala_deps~io_bazel_rules_scala_scala_compiler/BUILD
and referenced by '@@_main~_repo_rules~io_bazel_rules_scala//scala:default_toolchain_scala_compile_classpath_provider'
```

Also fixes the following error when attaching resources from custom repos to
targets under Bzlmod:

```txt
$ bazel test //test/src/main/scala/scalarules/test/resources:all

1) Scala library depending on resources from external resource-only
  jar::allow to load resources(scalarules.test.resources.ScalaLibResourcesFromExternalDepTest)
  java.lang.NullPointerException
    at scalarules.test.resources.ScalaLibResourcesFromExternalDepTest.get(ScalaLibResourcesFromExternalDepTest.scala:17)
    at scalarules.test.resources.ScalaLibResourcesFromExternalDepTest.$anonfun$new$3(ScalaLibResourcesFromExternalDepTest.scala:11)
    at scalarules.test.resources.ScalaLibResourcesFromExternalDepTest.$anonfun$new$2(ScalaLibResourcesFromExternalDepTest.scala:11)
```

Can be replaced with a future bazel-skylib implementation, if accepted
into that repo.

---

We can't rely on the specific canonical repository name format:

> Repos generated by extensions have canonical names in the form of
> `module_repo_canonical_name+extension_name+repo_name`. For extensions
> hosted in the root module, the `module_repo_canonical_name` part is
> replaced with the string `_main`. Note that the canonical name format is
> not an API you should depend on — it's subject to change at any time.
>
> - https://bazel.build/external/extension#repository_names_and_visibility

The change to no longer encode module versions in canonical repo names in
Bazel 7.1.0 is a recent example of Bazel maintainers altering the format:

- bazelbuild/bazel#21316

And the maintainers recently replaced the `~` delimiter with `+` in the
upcoming Bazel 8 release due to build performance issues on Windows:

- bazelbuild/bazel#22865

This function assumes the only valid `repo_name` characters are letters,
numbers, '_', '-', and '.'. It finds the last character not in this set, and
returns the contents of `name` following this character. This is valid so
long as this condition holds:

- https://github.com/bazelbuild/bazel/blob/7.3.2/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryName.java#L159-L162

scala/private/macros/bzlmod.bzl

@@ -0,0 +1,23 @@
+"""Utilities for working with Bazel modules"""
+
+def apparent_repo_name(label_or_name):
+    """Return a repository's apparent repository name.
+
+    Can be replaced with a future bazel-skylib implementation, if accepted into
+    that repo.
+
+    Args:
+        label_or_name: a Label or repository name string
+
+    Returns:
+        The apparent repository name
+    """
+    repo_name = getattr(label_or_name, "repo_name", label_or_name)
+
+    # Bazed on this pattern from the Bazel source:
+    # com.google.devtools.build.lib.cmdline.RepositoryName.VALID_REPO_NAME
+    for i in range(len(repo_name) - 1, -1, -1):
+        c = repo_name[i]
+        if not (c.isalnum() or c in "_-."):
+            return repo_name[i + 1:]
+    return repo_name

scala/private/resources.bzl

@@ -1,3 +1,5 @@
+load(":macros/bzlmod.bzl", "apparent_repo_name")
+
 def paths(resources, resource_strip_prefix):
     """Return a list of path tuples (target, source) where:
         target - is a path in the archive (with given prefix stripped off)
@@ -13,7 +15,13 @@ def paths(resources, resource_strip_prefix):
 
 def _target_path(resource, resource_strip_prefix):
     path = _target_path_by_strip_prefix(resource, resource_strip_prefix) if resource_strip_prefix else _target_path_by_default_prefixes(resource)
-    return _strip_prefix(path, "/")
+    return _update_external_target_path(_strip_prefix(path, "/"))
+
+def _update_external_target_path(target_path):
+    if not target_path.startswith("external/"):
+        return target_path
+    prefix, repo_name, rest = target_path.split("/")
+    return "/".join([prefix, apparent_repo_name(repo_name), rest])
 
 def _target_path_by_strip_prefix(resource, resource_strip_prefix):
     # Start from absolute resource path and then strip roots so we get to correct short path

scala/scala_maven_import_external.bzl

@@ -35,6 +35,7 @@ the following macros are defined below that utilize jvm_import_external:
 - java_import_external - to demonstrate that the original functionality of `java_import_external` stayed intact.
 """
 
+load("//scala/private:macros/bzlmod.bzl", "apparent_repo_name")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "read_netrc", "read_user_netrc", "use_netrc")
 
 # https://github.com/bazelbuild/bazel/issues/13709#issuecomment-1336699672
@@ -136,7 +137,7 @@ def _jvm_import_external(repository_ctx):
         "",
         "alias(",
         "    name = \"jar\",",
-        "    actual = \"@%s\"," % repository_ctx.name,
+        "    actual = \"@%s\"," % apparent_repo_name(repository_ctx.name),
         ")",
         "",
     ]))

third_party/repositories/repositories.bzl

@@ -1,3 +1,4 @@
+load("//scala/private:macros/bzlmod.bzl", "apparent_repo_name")
 load(
     "//third_party/repositories:scala_2_11.bzl",
     _artifacts_2_11 = "artifacts",
@@ -95,8 +96,10 @@ def repositories(
     default_artifacts = artifacts_by_major_scala_version[major_scala_version]
     artifacts = dict(default_artifacts.items() + overriden_artifacts.items())
     for id in for_artifact_ids:
+        generated_rule_name = apparent_repo_name(id) + suffix
         _scala_maven_import_external(
             name = id + suffix,
+            generated_rule_name = generated_rule_name,
             artifact = artifacts[id]["artifact"],
             artifact_sha256 = artifacts[id]["sha256"],
             licenses = ["notice"],
@@ -111,13 +114,13 @@ def repositories(
         # See: https://github.com/bazelbuild/rules_scala/pull/1573
         # Hopefully we can deprecate and remove it one day.
         if suffix and scala_version == SCALA_VERSION:
-            _alias_repository(name = id, target = id + suffix)
+            _alias_repository(name = id, target = generated_rule_name)
 
 def _alias_repository_impl(rctx):
     """ Builds a repository containing just two aliases to the Scala Maven artifacts in the `target` repository. """
 
     format_kwargs = {
-        "name": rctx.name,
+        "name": apparent_repo_name(rctx.name),
         "target": rctx.attr.target,
     }
     rctx.file("BUILD", """alias(