bazel: handle generated protos from external repos

When pyprotoc is used through an external
repository, generated proto files can arrive with
a path/short_path mismatch:

- path: bazel-out/.../external/<repo>/...
- short_path: ../<repo>/...

The previous logic in `rules.bzl` did not match
this shape and fell through to "Handling this type
of (generated?) .proto file was not forseen".

This commit adds a narrow branch in the existing
proto loop to normalize this short_path form to a
repo-relative path and emit the corresponding `-I`
+ input proto args.

This keeps existing behavior for local files,
external source files, and `_virtual_imports`,
while fixing generated protos from external repos.

Author: rjhuijsman

rules.bzl

@@ -101,6 +101,18 @@ def _protoc_plugin_rule_implementation(context):
     proto_files = unique_proto_files
 
     for proto_file in proto_files:
+        # Handle generated files in external repositories where Bazel surfaces
+        # short paths like "../<repo>/<repo-relative-path>.proto".
+        if proto_file.short_path.startswith("../"):
+            short_path = proto_file.short_path.removeprefix("../")
+            if "/" in short_path:
+                short_path = short_path.split("/", 1)[1]
+                if proto_file.path.endswith(short_path):
+                    import_path = proto_file.path[:-len(short_path)]
+                    args.append("-I" + import_path)
+                    args.append(short_path)
+                    continue
+
         if len(proto_file.owner.workspace_root) == 0:
             # Handle case where `proto_file` is a local file.
             # This includes both source files in the workspace and files