Add test showcasing issue with directory inputs

tests/ios/directory-input/BUILD.bazel

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+load("//tests/ios/directory-input:sources_gen_to_dir.bzl", "sources_gen_to_dir")
+load("//rules:framework.bzl", "apple_framework")
+load("@build_bazel_rules_swift//swift:swift.bzl", "swift_library")
+
+sources_gen_to_dir(
+    name = "gen_sources.swift",
+    output_dir = "gen_sources",
+)
+
+swift_library(
+    name = "LibraryFromGeneratedDirectory",
+    srcs = [":gen_sources.swift"],
+)
+
+apple_framework(
+    name = "FrameworkFromGeneratedDirectory",
+    srcs = [":gen_sources.swift"],
+)

tests/ios/directory-input/gen_sources/baz.swift

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+let bar = "bar"

tests/ios/directory-input/gen_sources/foo.swift

@@ -0,0 +1 @@
+let foo = "foo"

tests/ios/directory-input/sources_gen_to_dir.bzl

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+"""Example rule which uses `declare_directory` to declare a directory output fake Swift sources."""
+
+def _sources_gen_to_dir_impl(ctx):
+    directory = ctx.actions.declare_directory(ctx.attr.output_dir)
+    args = ctx.actions.args()
+    args.add(directory.path)
+
+    ctx.actions.run_shell(
+        arguments = [args],
+        command = """mkdir -p "$1" && echo 'let foo = "foo"' > $1/foo.swift && echo 'let bar = "bar"' > $1/baz.swift""",
+        inputs = [],
+        outputs = [directory],
+    )
+
+    return [
+        DefaultInfo(files = depset([directory])),
+    ]
+
+
+sources_gen_to_dir = rule(
+    implementation = _sources_gen_to_dir_impl,
+    attrs = {
+        "output_dir": attr.string(),
+    },
+)