Fix false positives in conversion due to prebulits
The prebuilt_ prefix was removed before writing converted modules, which
causes false positives when a prebuilt and from source module exist with
the same name but only one has been converted
Test: b run //build/bazel/scripts/bp2build_progress:bp2build_progress \
-- report -m core-lambda-stubs-for-system-modules
Change-Id: I8e972bf37983f8b19c76be0714bb0d23dbb0f60a