meta: support for inputs

Inputs are modules like Stages, Assemblers and Sources. Add them
as a new module klass to the various functions. Include them in
the schema test, so the schema of all inputs is validated.
Also sort the module classes alphabetically in the class mapping
and class list.

test/mod/test_osbuild.py

@@ -124,7 +124,7 @@ class TestDescriptions(unittest.TestCase):
         index = osbuild.meta.Index(os.curdir)
 
         modules = []
-        for klass in ("Stage", "Assembler", "Source"):
+        for klass in ("Assembler", "Input", "Source", "Stage"):
             mods = index.list_modules_for_class(klass)
             modules += [(klass, module) for module in mods]