Version requirements only work for the package index

[schlessera]

The current parsing code that tries to identify the package only allows for version constraints when using the package index. For git repositories, it just fails with an Error: Invalid package..

Ideally, the parsing should accept any string that the underlying Composer would be able to handle, with a fallback to building a proper GitHub URL for shortened syntax to account for the package index deprecation.

[danielbachhuber]

@schlessera Moving this out of the milestone, given there isn't a pull request for it yet. We can land whenever it's ready.

[schlessera]

@danielbachhuber The functionality was already in there since #22 was merged, due to the way I implemented that one. I hadn't closed the issue yet because I still had to create test cases to actually test all the scenarios.

All tests ran successfully, so this was working for some time now.