fix: plugin_output in py_proto_library rule (#1280)

plugin_output was wrong in case multiple repositories are involved
and/or _virtual_imports.

The code is taken from `cc_proto_library` and has been verified in
practice.

python/private/proto/py_proto_library.bzl

diff --git a/python/private/proto/py_proto_library.bzl b/python/private/proto/py_proto_library.bzl
index 9885585..9377c85 100644
--- a/python/private/proto/py_proto_library.bzl
+++ b/python/private/proto/py_proto_library.bzl
@@ -75,12 +75,22 @@
             name_mapper = lambda name: name.replace("-", "_").replace(".", "/"),
         )
 
+        # Handles multiple repository and virtual import cases
+        proto_root = proto_info.proto_source_root
+        if proto_root.startswith(ctx.bin_dir.path):
+            plugin_output = proto_root
+        else:
+            plugin_output = ctx.bin_dir.path + "/" + proto_root
+
+        if plugin_output == ".":
+            plugin_output = ctx.bin_dir.path
+
         proto_common.compile(
             actions = ctx.actions,
             proto_info = proto_info,
             proto_lang_toolchain_info = proto_lang_toolchain_info,
             generated_files = generated_sources,
-            plugin_output = ctx.bin_dir.path,
+            plugin_output = plugin_output,
         )
 
     # Generated sources == Python sources