lib/modules: Change mkAliasOptionModule to use the priority for the alias.

This commit changes the `mkAliasOptionModule` function to make sure that
the priority for the aliased option is propagated to the non-aliased
option.

This also affects the `mkRenamedOptionModule` function in a similar
fashion.

This also removes the `mkAliasOptionModuleWithPriority` function, since
its functionality is now subsumed by `mkAliasOptionModule`.

This change was recommended by @nbp:
https://github.com/NixOS/nixpkgs/pull/53397#discussion_r245487432

lib/tests/modules/alias-with-priority-can-override.nix

@@ -32,7 +32,7 @@ with lib;
 
   imports = [
     # Create an alias for the "enable" option.
-    (mkAliasOptionModuleWithPriority [ "enableAlias" ] [ "enable" ])
+    (mkAliasOptionModule [ "enableAlias" ] [ "enable" ])
 
     # Disable the aliased option, but with a default (low) priority so it
     # should be able to be overridden by the next import.

lib/tests/modules/alias-with-priority.nix

@@ -32,7 +32,7 @@ with lib;
 
   imports = [
     # Create an alias for the "enable" option.
-    (mkAliasOptionModuleWithPriority [ "enableAlias" ] [ "enable" ])
+    (mkAliasOptionModule [ "enableAlias" ] [ "enable" ])
 
     # Disable the aliased option, but with a default (low) priority so it
     # should be able to be overridden by the next import.