Allow backend packages to also register remote auth plugins

Follow up for pantsbuild#16212

This is useful when the remote auth plugin is a first party (in repo plugin).
also add some debug logging.

src/python/pants/init/extension_loader.py

@@ -2,6 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 import importlib
+import logging
 import traceback
 from typing import Dict, List, Optional
 
@@ -12,6 +13,8 @@
 from pants.goal.builtins import register_builtin_goals
 from pants.util.ordered_set import FrozenOrderedSet
 
+logger = logging.getLogger(__name__)
+
 
 class PluginLoadingError(Exception):
     pass
@@ -100,6 +103,9 @@ def load_plugins(
             build_configuration.register_rules(req.key, rules)
         if "remote_auth" in entries:
             remote_auth_func = entries["remote_auth"].load()
+            logger.debug(
+                f"register remote auth function {remote_auth_func.__module__}.{remote_auth_func.__name__} from plugin: {plugin}"
+            )
             build_configuration.register_remote_auth_plugin(remote_auth_func)
 
         loaded[dist.as_requirement().key] = dist
@@ -137,7 +143,7 @@ def load_backend(build_configuration: BuildConfiguration.Builder, backend_packag
         traceback.print_exc()
         raise BackendConfigurationError(f"Failed to load the {backend_module} backend: {ex!r}")
 
-    def invoke_entrypoint(name):
+    def invoke_entrypoint(name: str):
         entrypoint = getattr(module, name, lambda: None)
         try:
             return entrypoint()
@@ -156,3 +162,9 @@ def invoke_entrypoint(name):
     rules = invoke_entrypoint("rules")
     if rules:
         build_configuration.register_rules(backend_package, rules)
+    remote_auth_func = getattr(module, "remote_auth", None)
+    if remote_auth_func:
+        logger.debug(
+            f"register remote auth function {remote_auth_func.__module__}.{remote_auth_func.__name__} from backend: {backend_package}"
+        )
+        build_configuration.register_remote_auth_plugin(remote_auth_func)