#2990 stamped definition_file with the still-absolute implementation path, so graph.json
shipped the build host's absolute path beside a repo-relative source_file (and the AST cache
replayed it on every warm rebuild). Normalize definition_file through the same
_norm_source_file path as source_file in build_from_json and the cache round-trip, so it is
relative/portable and byte-stable across machines.
