Improve handling of extras

[maksbotan]

Pull Request Check List

This changeset improves caching of dependencies with extras in two ways:

• foo[extra] -> foo dependency preserves information about foo's source. If it comes from secondary repo, poetry will not look for it on PyPI

• Do not resolve optional

• [ ] Added tests for changed code.

• [ ] Updated documentation for changed code.

I'm all in for adding tests, but I'd need guidance how to proceed.

When this PR is merged, I'll backport it to 1.1 branch as well.