osbuild: use load function via the format module

Instead of importing the load, load_build functions into the osbuild
namespace and using it via that, use the load function via the module
that provides them, i.e. the formats.v1 module.

test/mod/test_osbuild.py

@@ -31,7 +31,7 @@ class TestDescriptions(unittest.TestCase):
         ]
         for pipeline in cases:
             with self.subTest(pipeline):
-                self.assertEqual(osbuild.load(pipeline, {}).description(), {})
+                self.assertEqual(fmt.load(pipeline, {}).description(), {})
 
     def test_stage(self):
         name = "org.osbuild.test"