[ssci] Return relative license filepath instead of absolute

This CL updates the third party metadata field License File, so that
missing license filepaths are reported as relative to the repo root,
instead of the absolute filepath.


Bug: b:277147404
Change-Id: I16ac23d86b0ef12ab6020d8e81525965489888ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4876394
Auto-Submit: Anne Redulla <aredulla@google.com>
Reviewed-by: Rachael Newitt <renewitt@google.com>
Commit-Queue: Anne Redulla <aredulla@google.com>

metadata/fields/custom/license_file.py

@@ -110,7 +110,8 @@ class LicenseFileField(field_types.MetadataField):
                     source_file_dir, os.path.normpath(license_filename))
 
             if not os.path.exists(license_filepath):
-                invalid_values.append(license_filepath)
+                rel_filepath = os.path.relpath(license_filepath, repo_root_dir)
+                invalid_values.append(rel_filepath)
 
         if invalid_values:
             missing = ", ".join(invalid_values)