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.go b/bp2build/conversion.go
index 7791419..35a1400 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -48,7 +48,11 @@
 	}
 	files = append(files, newFile("apex_toolchain", "constants.bzl", apexToolchainVars))
 
-	files = append(files, newFile("metrics", "converted_modules.txt", strings.Join(metrics.Serialize().ConvertedModules, "\n")))
+	if buf, err := json.MarshalIndent(metrics.convertedModuleWithType, "", "  "); err != nil {
+		return []BazelFile{}, err
+	} else {
+		files = append(files, newFile("metrics", "converted_modules.json", string(buf)))
+	}
 
 	convertedModulePathMap, err := json.MarshalIndent(metrics.convertedModulePathMap, "", "\t")
 	if err != nil {