py_proto_library: external runfiles fix

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.

This is identical to the change made to rules_python in bazelbuild/rules_python#2516.

Work towards bazelbuild/rules_python#2581.

PiperOrigin-RevId: 721941173

bazel/py_proto_library.bzl

@@ -76,7 +76,12 @@ 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_root = proto_root[len("external") + 1:]
+        else:
+            proto_root = ctx.workspace_name + "/" + proto_root
 
         proto_common.compile(
             actions = ctx.actions,