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
diff --git a/bp2build/conversion_test.go b/bp2build/conversion_test.go
index cbffaa0..89dd38e 100644
--- a/bp2build/conversion_test.go
+++ b/bp2build/conversion_test.go
@@ -134,7 +134,7 @@
 		},
 		{
 			dir:      "metrics",
-			basename: "converted_modules.txt",
+			basename: "converted_modules.json",
 		},
 		{
 			dir:      "metrics",