eased the redability of the installable decorator

plugins/module_utils/module.py

@@ -323,6 +323,8 @@ _INSTALL_MAPPING = dict(
 
 def installable(_class: Type[SystemdUnitModule]):
     """adds the required arguments to the spec and adds the install method for the unit method"""
+    specs = _class.module_spec
+
     arguments = dict(
         required_by=Types.list(elements=str, help="systemd units that require this mount"),
         wanted_by=Types.list(
@@ -330,7 +332,7 @@ def installable(_class: Type[SystemdUnitModule]):
             help="systemd units that want the mount, but not explicitly require it. Commonly used for target if not service explicitly require it.",
         ),
     )
-    _class.module_spec["argument_spec"].update(arguments)
+    specs["argument_spec"].update(arguments)
 
     def install(self: SystemdUnitModule) -> str:
         output = "[Install]\n"
@@ -341,4 +343,5 @@ def installable(_class: Type[SystemdUnitModule]):
         return output
 
     _class.install = install
+    _class.module_spec = specs
     return _class