gclient_utils.py is a kitchen sink and is for that reason expensive
to import. Move the comparatively cheap and simple path routines
to a new gclient_paths module and use that in gn.py, clang_format.py,
dart_format.py.

(To be able to move FindGclientRoot() to gclient_paths.py,
make it use io.open() instead of FileRead(). FileRead() tries
to paper over invalid utf-8, but that was added for presubmits,
not for .gclient files, so this is hopefully fine.)

Cuts gn.py overhead in half (on my Windows laptop from 0.6s to 0.25s,
still high; on my Mac laptop from 0.1s to 0.05s), and probably helps
the other two too.

Completely remove PathDifference() since it's unused.

Bug: 939959
Change-Id: I6a70f6e4c16062b622fb2df8778e8a598d4cc956
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1512058
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>