Fix long-standing bugs with MANIFEST.in parsing on Windows (#6884).

Fix long-standing bugs with MANIFEST.in parsing on Windows (#6884).

These regex changes fix a number of issues for distutils on Windows:
- #6884: impossible to include a file starting with 'build'
- #9691 and #14004: sdist includes too many files
- #13193: test_filelist failures

This commit replaces the incorrect changes done in 557a973709de,
c566a3447ba1 and 3925081a7ca0 to fix #13193; we were too eager to fix
the test failures and I did not study the code enough before
greenlighting patches.  This time we have unit tests from the problems
reported by users to be sure we have the right fix.

Thanks to Nadeem Vawda for his help.