mpdecimal: Fix lib name on MSVC/shared, fix Mac cross-builds

[Ahajha]

When using the MSBuildDeps generator with mpdecimal(msvc/shared), it generates the wrong lib name and thus fails to build. I assume CMake does some more advanced logic and so this was never caught.

The correct name is (for example) libmpdec-2.5.1.dll.lib, previously this was just libmpdec-2.5.1.lib.

This change is necessary for supporting shared mpdecimal on Windows in CPython.

Also fixes #23994 (this is fixed by both of my other open mpdecimal PRs, since it's necessary to build. First one to be merged will fix it)

I also have two other mpdecimal PRs out: #25896 #25903 (superset of this one)

[jcar87]

I also added some extra cleanup commits, none of these are necessary and I'm happy to revert any of them if it makes it easier to review the PR

yes please - this makes it a lot easier to review, especially with the size of the backlog - thanks!

[Ahajha]

@jcar87 I reverted the extra changes, diff is much smaller now :) I'll make a followup PR after the important 3 are merged to just do all the cleanup.

This PR is now a subset of #25903, so feel free to either skip this and just review that one, or merge this first and then the other one.

[jcar87]

LGTM, thanks!