This change allows switching out the url type for requirements. The
original idea was to allow different types for different requirement
origins, so that core metadata reads can ban non-pep 508 requirements
while we only allow them for requirements.txt. This didn't work out
because we expect `&Requirement`s from all sources to match.
I also tried to split `pep508_rs` into a PEP 508 compliant crate and
into our extensions, but they are to tightly coupled.
I think this change is an improvement still as it reduces the hardcoded
dependence on `VerbatimUrl`.
konsti