Reorganizes the build files to work with a more divided setup. It is
unclear if we'll ever be able to run tests in a UWP config. This at
least sets up the organization so it would at some point be possible.

Bug: angleproject:4182
Change-Id: I49dddfcdc0118b11466fe171f949c28d101ac6a2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1953484
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>