Summary
Fixes #2822

When using file: dependencies yarn ignores the files reference in the package.json and copies over the whole folder. This causes all kinds of issues for folks. See #2822 and related for the various ways it plays havok.

This change addresses that by checking the package for a files array and if present only copying the files found there, along with the package.json file itself. I did update the broken contributing links mentioned below as well but there is another pr with those changes so I dropped those

Test plan
Added several new tests to fetchers.js. For a concrete test and repro see the original issue here: #2822

Notes
I'm new to the codebase so I had a few notes/questions.

1. should I be hardcoding package.json? I saw a few examples of this in the git-fetcher so figured it's ok but it seems like I should ask the manifest for its _filename or something.
2. I'd love to refactor the fs.copy command to correctly handle sources as files and destinations as directories as is idiomatic but given this is a first PR and that seems core I simply made sure I was copying files to files here.
3. Ideally we should be following a similar pattern to the git-fetcher and packing and unpacking to get full ignore and build support. Happy to refactor to that next but figured it might be a bit much for this change.
4. Generally, this seems like a pretty straightforward issue. Any ideas why none of the commenters submitted a PR over the three years it's been open? Other than a broken link or two, so far, contributing to yarn seems fairly frictionless.