[5.x] bzlmod: Add support for WORKSPACE.bzlmod (bazelbuild#14813)

When enabling Bzlmod, if a WORKSPACE.bzlmod file is presented at the
source root, it will replace the content of the WORKSPACE file.
In contrast to the WORKSPACE file, no prefix or suffix are added to the WORKSPACE.bzlmod file.

This file is for migration purpose in case some of the dependencies
still need to be introduced via the WORKSPACE mechanism, eventually
it should be removed.

RELNOTES: None
PiperOrigin-RevId: 407802717

Author: meteorcloudy