This is somewhat tricky to implement because path equality is not
straightforward. See
https://github.com/abseil/abseil-cpp/pull/1560#issuecomment-1799983471
for discussion.

This re-lands 3bd86026 with a fix
for iOS 13 unavailability of std::filesystem::path.

Roll-forward of 524ebb7e.

Closes #655
Closes #1560

PiperOrigin-RevId: 583365100
Change-Id: Id49735c49d123e0cd6a620a2b5b5a12d94129f94