Bug 1272976 - Don't import anything from the mozwebidlcodegen module globally; r=gps

This import makes out-of-tree builds of SpiderMonkey depend on
dom/bindings/mozwebidlcodegen needlessly.

python/mozbuild/mozbuild/backend/common.py

@@ -9,7 +9,6 @@ import json
 import os
 
 import mozpack.path as mozpath
-import mozwebidlcodegen
 
 from mozbuild.backend.base import BuildBackend
 
@@ -423,6 +422,8 @@ class CommonBackend(BuildBackend):
         with self._write_file(file_lists) as fh:
             json.dump(o, fh, sort_keys=True, indent=2)
 
+        import mozwebidlcodegen
+
         manager = mozwebidlcodegen.create_build_system_manager(
             self.environment.topsrcdir,
             self.environment.topobjdir,