Use absolute urls in `distribution_type::File` (#917)
Previously, the url on file could either be a relative or an absolute
url, depending on the index, and we would finalize it lazily. Now we
finalize the url when converting `pypi_types::File` to
`distribution_types::File`. This change is required to make the hashes
on `File` optional (https://github.com/astral-sh/puffin/pull/910), which
are currently the only unique field usable for caching.
