Fix false pos in bp2build-prog 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: Id00099780fb6af9fffcf745b509116a66bac8756