underlying lib

This is a rollforward of
bazelbuild#2803, but behind a feature
flag.

This PR also makes `rust_test` put its compilation outputs in the same
directory as the `rust_library` rule (i.e. not in a `test-{hash}`
subdirectory anymore).

After this change both the `rust_library` and `rust_test` rules will put
all its compilation outputs in the same directory, but there won't be
any name collisions in non-sandboxed environments (see
bazelbuild#1427 for more context).

Issue: bazelbuild#2827

…t's True

…nd-change-rust-test-compilation-output-dir