Implement the new resolver's Requirement class

[pfmoore]

This is the initial implementation of the new resolver's Requirement class. It isn't used anywhere in pip yet, further PRs will add the other resolver classes and link them together.

The tests here are very basic "does this thing work?" tests, more detailed functionality tests will be added when the full class structure is present.

[pfmoore]

Ping @pradyunsg @uranusjr. Getting this in here for review. I think it's good enough to go.

Candidate will be coming soon, but I want to do some tidying up and simplification before creating a PR.

[uranusjr]

Are there cases where name wouldn’t be called? I can’t think of any. I am tempted to suggest always NamedRequirement instead, by getting the candidate very eagerly in make_requirement and assigning it to the InstallRequirement.

[uranusjr]

I.e. something like

def make_requirement(install_req):
    # type: (InstallRequirement) -> Requirement
    if not install_req.link:
        return VersionedRequirement(install_req)
    if not install_req.req:
        install_req.req = _parse_link_into_req(install_req.link)  # TODO: Implement me.
    if install_req.req.name:
        return NamedRequirement(install_req, candidate=None)
    candidate = _make_candidate(install_req)  # TODO: Implement me.
    install_req.req.name = candidate.name
    return NamedRequirement(install_req, candidate)

[pfmoore]

Agreed, but without a "proper" candidate object, I can't do this without the tests failing (see the test that currently asserts that a sdist has a name of ""). So I plan on addressing this in one of the follow up PRs (either the one that implements candidates, or more likely a further PR that tidies up all the links).

In hindsight, I'm not convinced that splitting up the original change from #7799 was worthwhile, it's created a lot of busy-work, and I don't think the resulting PRs are that much more reviewable. But adding the tests was (IMO) a definite benefit, so I'm not going to worry too much..

[uranusjr]

On the ground that we need to get something in to get things started.