Provide "official" pkg_filegroup-using RPM rule and tests (#303)

This change replaces the experimental `pkg_rpm` rule in
`pkg/experimental/rpm.bzl` rule with one that is aware of the new
`pkg_filegroup` semantics in `pkg/mappings.bzl`, with the following additional
changes:

- The `%files` metadata build-out is made more clear.  Additionally, the
  `rpm_filetag` mapping rule attribute replaces the `section` value previously
  passed to `pkg_filegroup`.

- Inputs to `pkg_rpm` are now specified with the `srcs` attribute instead of
  `data`, to be consistent with regards to the nameing of runtime requirements
  and build-time sources.  This also matches the implementation of `pkg_tar` and
  `pkg_zip`; `pkg_deb` stilluses `data`.  These should all be made consistent at
  some point.

- Conflicting package contents detection and error logging is now streamlined.

- Test coverage has improved, including new analysis tests for conflict
  detection.  Also, bugs in the RPM python tests were fixed opportunistically.

The old `pkg_filegroup` code in this same directory will be removed in a
separate change.

Fixes to other known issues with this implementation of `pkg_rpm` were not
attempted, and are currently tracked in #300.

pkg/experimental/BUILD

@@ -12,13 +12,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-licenses(["notice"])
+load("//:mappings.bzl", "pkg_filegroup", "pkg_files")
+load(":rpm.bzl", "pkg_rpm")
 
 exports_files(
     glob([
         "*.bzl",
+    ]) + [
         "template.spec.in",
-    ]),
+    ],
     visibility = ["//visibility:public"],
 )
 
@@ -32,3 +34,23 @@ filegroup(
     ],
     visibility = ["//:__pkg__"],
 )
+
+pkg_files(
+    name = "artifacts",
+    srcs = [":template.spec.in"],
+)
+
+pkg_filegroup(
+    name = "usr_share_artifacts",
+    srcs = [":artifacts"],
+)
+
+pkg_rpm(
+    name = "example",
+    srcs = [":usr_share_artifacts"],
+    description = "Other example!",
+    license = "N/A",
+    release = "1",
+    summary = "Example!",
+    version = "1.0",
+)