Have duplicate metadata and invalid req strings honor --warn option

[kemzeb]

This resolves #355 by making changes and refactors to the warning logic. It does so by introducing a module-level singleton "WarningPrinter" object and refactors the code in such a way to integrate this object for it to be used.

[kemzeb]

Still working on this, I'll see if I can finish this and open it for review by the end of today.

[kemzeb]

Did a redesign and this seems to be a better implementation than the last. Just need to add coverage tests then I'll open this PR for review

[kemzeb]

I think this is ready to go, going to open this for review

[kemzeb]

Here are my design decisions:

• Decided to keep the warning printing logic in their respective modules since they appear to be tightly coupled together
• Included a _warning module; did try to add it to render but I encountered dependency cycle errors
• Implemented a WarningPrinter class, where I also exposed a shared module-level singleton of it
• Decided that the duplicate metadata warning should not be considered a "fail" warning when --warn fail is passed. My reasoning for this is that Python allows these duplicate distributions to exist and pip actively ignores them.
• I thought it would be nice for WarningPrinter.print_multi_line() to pass what file to write to to the callback so that we avoid throwing sys.stderr everywhere, but due to an odd long-standing bug with the capsys/capfd fixtures, it won't properly capture sys.stderr if we were to pass it down to the callback.

[xiacunshun]

Ignore my previous deleted comment, I've reconsidered and the handling of "duplicated" here should be fine.

[xiacunshun]

No questions for me here. But I don't have the approve auth, so maybe need to wait for @gaborbernat to take a further look.

[kemzeb]

Great, will release 2.19.0 after merging this