tests: refactor and fix repository fixtures

This change introduces httpretty based mocking for tests using
mocked legacy or pypi repositories. This also fixes various issues with
tests incorrectly relying on remote versions for metadata and
distributions instead of expected mocked versions.

Additionally, we also auto enables PyPI repository mocking to ensure no
external requests are made without explicit handling.

Further, a new `generate.py` script has now been added to help
regenerate existing repository json and distribution fixtures; or
add/remove release versions to the fixtures.