Tidy up manylinux tests and musllinux manifest

[uranusjr]

Unifying code structuring differences between manylinux and musllinux so they can be maintained more easily together.

1. Manylinux logic is separated into a private module _manylinux like musllinux. This makes packaging.tags a bit more maintainable, it was almost 800 lines previously.
2. Some manylinux tests are split into test_manylinux.py for the same reason.
3. Binaries used in manylinux tests are moved into a subdirectory (similar to how musllinux tests’ binaries are located); previously they were pretty randomly placed in the tests’ root directory.
4. Tidy up MANIFEST.in to include musllinux tests’ binaries in the sdist.

[brettcannon]

We should probably get this merged before we cut a 21.0 release.

[brettcannon]

Thanks!