meta: extract module class to dir mapping

Make the mapping of module class to the corresponding directory
a method of the ModuleInfo class. This is so it can be re-used
by others in the future.

osbuild/meta.py

@@ -326,16 +326,10 @@ class ModuleInfo:
         def targets(a):
             return [t.id for t in filter_type(a.targets, ast.Name)]
 
-        mapping = {
+        base = cls.module_class_to_directory(klass)
-            "Stage": "stages",
+        if not base:
-            "Assembler": "assemblers"
-        }
-
-        if klass not in mapping:
             raise ValueError(f"Unsupported type: {klass}")
 
-        base = mapping[klass]
-
         path = os.path.join(root, base, name)
         try:
             with open(path) as f:
@@ -349,6 +343,15 @@ class ModuleInfo:
         info = {k: value(v) for k, v in targets if k in names}
         return cls(klass, name, info)
 
+    @staticmethod
+    def module_class_to_directory(klass: str) -> str:
+        mapping = {
+            "Stage": "stages",
+            "Assembler": "assemblers"
+        }
+
+        return mapping.get(klass)
+
 
 class Index:
     """Index of stages and assemblers