Reenables b31414bf4f9898f7817a9fcf8a91f62ec26f3eaf.
Also adds a new warning for missing `--symbol-graph-dir` arg when
`--emit-extension-symbol-graphs` is provided. This also reverts the
commit that removed.
This extends ExtractAPI to take into account symbols defined in categories to types defined in an external module.
This introduces 2 new command line flags, `--symbol-graph-dir=DIR` and `--emit-extension-symbol-graphs`, when used together this generates additional symbol graph files at `DIR/ExtendedModule@ProductName.symbols.json` for each external module that is extended in this way.
Additionally this makes some cleanups to tests to make them more resilient and cleans up the `APISet` data structure.
