fix: normalize stub_path in repl.bzl (bazel-contrib#3104)

When a REPL target is run from an external Bazel module, the `stub_path`
can have path components in it (e.g. "/..") which get rejected by the
`Rlocation()` function in `runfiles.py` for not being normalized. This
commit normalizes the path before it's passed to `Rlocation()`.

Fixes bazel-contrib#3101

---------

Co-authored-by: Ignas Anikevicius <[email protected]>

Author: jpwoodbu

CHANGELOG.md

@@ -93,6 +93,8 @@ END_UNRELEASED_TEMPLATE
   ([#3099](https://github.com/bazel-contrib/rules_python/issues/3099)).
 * (pypi) Expose pypi packages only common to all Python versions in `all_requirements`
   ([#2921](https://github.com/bazel-contrib/rules_python/issues/2921)).
+* (repl) Normalize the path for the `REPL` stub to make it possible to use the
+  default stub template from outside `rules_python` ({gh-issue}`3101`).
 
 {#v0-0-0-added}
 ### Added

python/private/repl_template.py

@@ -5,7 +5,9 @@
 
 from python.runfiles import runfiles
 
-STUB_PATH = "%stub_path%"
+# runfiles.py will reject paths which aren't normalized, which can happen when the REPL rules are
+# used from a remote module.
+STUB_PATH = os.path.normpath("%stub_path%")
 
 
 def start_repl():