✨ Add VCS support

[ddelange]

Closes #61

Adds support for VCS and PEP 440 direct references:

pipgrip "requests[socks] @ git+https://github.com/psf/requests"
pipgrip "requests[socks] @ https://github.com/psf/requests/releases/download/v2.27.1/requests-2.27.1-py2.py3-none-any.whl"

• Treat a VCS requirement (parsed by pkg_resources into the url attr) as a unique identifier, as pip also doesn't perform resolving on different VCS urls that would resolve to the same code.
• As pkg_resources only successfully parses VCS when the @ is present, this is not edge-cased in this PR. It is noted in the known caveats section.
• pkg_resources fails to parse @ -e svn+. It is not edge-cased in this PR. It is noted in the known caveats section.
• Adding support for (local) paths/URLs other than . is not in scope of this PR