This PR implements the change discussed at https://docs.google.com/document/d/1EArrWYUDugqJzBcb0-OxY5BH1FFPYV3jxLIXbJpD9RY/edit?pli=1#heading=h.5mcn15i0e1ch.

Closes https://github.com/bazelbuild/bazel/pull/8600.
https://github.com/bazelbuild/bazel/issues/6516

This is encore of https://github.com/bazelbuild/bazel/commit/324dc44e6bafb487331724ae83d67bc18ed8a8aa with these changes:

* removing WORKSPACE file from the platforms_archive - repository rules always create new WORKSPACE file and that messes up the timestamps after re-archiving. This fixes the determinism test.
* after @aehlig kindly explained the semantics of external repositories, I discovered that overriding @platforms doesn't work when there is a load statement somewhere above the override, which is the common case. Therefore I had to move bundled platforms to the workspace suffix and had to use maybe pattern.
* because of maybe pattern I have to mock the bzl file in Bazel unit tests.

RELNOTES:
PiperOrigin-RevId: 253193463