Validate_upload should require contributors and antibodies to be actual files

[jswelling]

The current code apparently accepts '.' as contributors_path and presumably antibodies_path- at least, metadata.tsv files have passed validation with that value. The path for the contributors and antibodies tsv files should be that of an actual file, not a directory.

The relevant code seems to be here:

ingest-validation-tools/src/ingest_validation_tools/upload.py

Line 277 in b359ac2

def __get_antibodies_references(self) -> dict:

but it's unclear why it accepts a directory path without comment. The actual contents of the file are parsed elsewhere so maybe the ball is getting dropped in between the two.