sources: give each filename from a source a unique filename

sources: give each filename from a source a unique filename

To prevent dataset files from difference sources from overwriting each
other, give each file downloaded and extracted a prefix based on the
URL (a hash). This ensures unique filenames across all rulesets.

This mostly matters for datasets, as when datasets are processed we
are working with a merged set of filenames, unlike rules which are
parsed much earlier when we still have a list of files.

Not the most elegant solution, but saves a rather large refactor.

Bug: #6833