While adding the extrainterpreters package as a test case, I discovered
a bug where the regex we use for matching sdist and wheel filenames is
not correctly parsing certain prerelease versions. In this commit I'm
fixing that by outsourcing the parsing of wheel filenames to the package
wheel_filename, and writing a more accurate regex to parse sdist
filenames. I've added a tuple representing the components of an sdist
filename to parallel the structure that wheel_filename uses.

The new dependency is only needed in testing so it's relatively low cost
to add it.

This commit adds some tests of the sdist/wheel filename matching
function added in the previous commit. It's not terribly valuable to
have a lot of tests of wheel filename matching because it's being
handled by a separate package which (presumably) has its own tests,
but it'll be good to have tests regardless in case we ever switch to
a different implementation, and it costs us very little to add them.