Fix nested local reference resolution

[Guflly]

Resolves nested local $ref paths from the file that declares them instead of the root schema. Adds an acceptance test with a nested schema directory.

Local checks: 606 tests passed, 20 skipped; mypy and pre-commit also pass.

Fixes #640

[sirosen]

Hi, thanks for this! It looks more correct than the last fix someone sent, but I don't quite understand why it should work.

The current reference registry is built such that it's supposed to fallback to the retrieval URI if the $id is not present. I'm not clear on why it's not working in this case, but I'd rather that we fix it than start mutating the schema being evaluated.

Do you know why this is effective, when the lines here are meant to have a similar effect already?