refactor requirement preparer to remove duplicated code paths for metadata-only dists

[cosmicexplorer]

This PR is on top of #12863; see the +181/-153 diff against it at https://github.com/cosmicexplorer/pip/compare/dist-caching-changes...cosmicexplorer:pip:refactor-requirement-preparer?expand=1.

Problem

There is a lot of duplicated code in RequirementPreparer for the metadata-only and "normal" code paths. Regarding @pfmoore's analysis from #12186 (comment):

I'm concerned that the preparer is one of the least "obvious" parts of the resolution process - it's full of generic terms (not least of which is "prepare" itself!) which don't really give much clue as to what precisely is going on. Every time I need to deal with the preparer, I have to do a bunch of research to remind myself what's happening before I can proceed. I'm worried that if we delegate chunks of the "prepare" task to individual commands, we will end up making that problem worse, with everyone needing to keep track of what's happening.

Solution

• Split out a lot of logic from _prepare_linked_requirement() into private helper methods.

Result

Less duplicated code, and hopefully RequirementPreparer will be easier to maintain.