Summary:
getdeps.py's find_existing_win32_subst_for_path function tries to reuse an existing build directory alias. (On Windows, the build directory is aliased to a drive letter to shorten paths.) If this function does not find and existing build directory alias, getdeps.py invalidates many of its caches.

On my Windows machine, find_existing_win32_subst_for_path always fails, so all of my builds are super slow. This happens because find_existing_win32_subst_for_path is given a path with a lower-case drive letter ("c:\users\..."), but the subst command returns paths with an upper-case drive letter ("C:\users\...").

When comparing paths, use ntpath.normpath. This makes the comparison case-insensitive for drive letters. (It also makes the comparison case-insensitive for other path components.)

On Linux and macOS, this diff should not change behavior.

Reviewed By: wez

Differential Revision: D15466096

fbshipit-source-id: 1669aa0a6eaeb6012154f3a9e24eba3f835262c6