/etc/portage/modules: warn about metadata_overlay

This module is an alias for flat_hash now, and hasn't been useful
since FEATURES=metadata-transfer was disabled by default in commit
81db3e467cd7f8bb94fcdf96d5fb4acf27bf382b.
diff --git a/pym/portage/package/ebuild/config.py b/pym/portage/package/ebuild/config.py
index 542cfac..d49ac60 100644
--- a/pym/portage/package/ebuild/config.py
+++ b/pym/portage/package/ebuild/config.py
@@ -309,12 +309,19 @@
 
 			self.module_priority    = ("user", "default")
 			self.modules            = {}
-			modules_loader = KeyValuePairFileLoader(
-				os.path.join(config_root, MODULES_FILE_PATH), None, None)
+			modules_file = os.path.join(config_root, MODULES_FILE_PATH)
+			modules_loader = KeyValuePairFileLoader(modules_file, None, None)
 			modules_dict, modules_errors = modules_loader.load()
 			self.modules["user"] = modules_dict
 			if self.modules["user"] is None:
 				self.modules["user"] = {}
+			user_auxdbmodule = \
+				self.modules["user"].get("portdbapi.auxdbmodule")
+			if user_auxdbmodule is not None and \
+				user_auxdbmodule in self._module_aliases:
+				warnings.warn("'%s' is deprecated: %s" %
+				(user_auxdbmodule, modules_file))
+
 			self.modules["default"] = {
 				"portdbapi.auxdbmodule":  "portage.cache.flat_hash.database",
 			}