fix: py_proto_library: external runfiles

Previously, the import path within the runfiles was only correct for the
case --legacy_external_runfiles=True (which copied the runfiles into
`$RUNFILES/<main repo>/external/<external repo>/<path>` in addition to
`$RUNFILES/<external repo>/<path>`. This flag was flipped to False in
Bazel 8.0.0.

Fixes #2515.

Tested locally against the minimal reproducer in that issue.

Author: tpudlik

CHANGELOG.md

@@ -70,6 +70,7 @@ Unreleased changes template.
 * (pypi) Using {bzl:obj}`pip_parse.experimental_requirement_cycles` and
   {bzl:obj}`pip_parse.use_hub_alias_dependencies` together now works when
   using WORKSPACE files.
+* (py_proto_library) Fix import paths in Bazel 8.
 
 [pep-695]: https://peps.python.org/pep-0695/
 

python/private/proto/py_proto_library.bzl

@@ -98,7 +98,12 @@ def _py_proto_aspect_impl(target, ctx):
             proto_root = proto_root[len(ctx.bin_dir.path) + 1:]
 
         plugin_output = ctx.bin_dir.path + "/" + proto_root
-        proto_root = ctx.workspace_name + "/" + proto_root
+        # Import path within the runfiles tree
+        if proto_root.startswith('external/'):
+            proto_import_path = proto_root[len('external') + 1:]
+        else:
+            proto_import_path = ctx.workspace_name + "/" + proto_root
+
 
         proto_common.compile(
             actions = ctx.actions,
@@ -130,7 +135,7 @@ def _py_proto_aspect_impl(target, ctx):
                 # _virtual_imports. But it's undesirable otherwise, because it
                 # will put the repo root at the top of the PYTHONPATH, ahead of
                 # directories added through `imports` attributes.
-                [proto_root] if "_virtual_imports" in proto_root else [],
+                [proto_import_path] if "_virtual_imports" in proto_import_path else [],
                 transitive = [dep[PyInfo].imports for dep in api_deps] + [dep.imports for dep in deps],
             ),
             runfiles_from_proto_deps = runfiles_from_proto_deps,