Add support for mkosi.clean.d and drop mkosi.configure.d

It makes sense to have multiple clean scripts, but with configuration we expect
a single output like with version and rootpw

mkosi/config.py

@@ -2024,7 +2024,6 @@ def parse_ini(path: Path, only_sections: Collection[str] = ()) -> Iterator[tuple
         section="Config",
         parse=config_make_list_parser(delimiter=",", parse=make_path_parser()),
         paths=("mkosi.configure",),
-        recursive_paths=("mkosi.configure.d/",),
         help="Configure script to run before doing anything",
     ),
     ConfigSetting(
@@ -2254,6 +2253,7 @@ def parse_ini(path: Path, only_sections: Collection[str] = ()) -> Iterator[tuple
         section="Output",
         parse=config_make_list_parser(delimiter=",", parse=make_path_parser()),
         paths=("mkosi.clean",),
+        recursive_paths=("mkosi.clean.d",)
         help="Clean script to run after cleanup",
     ),
     # Content section