rust: Suppress default sysroot unconditionally

With proper prebuilt modules, we can avoid any rustc implicit sysroot
searching.

Asd a bonus, this should make rust-project.json generation correctly
grab otherwise implicit dependencies.

Prebuilt rlibs may include several dependency rlibs. Without a
link_dirs attribute, every dependency (even if unexported) would need a
separate module.

Previously we were casing out on exact structs, which might be OK when
libraryDecorator and procMacroDecorator were the only possibilities, but
repeating the logic for three types is too much. Using an interface
makes this logic scale better.

Bug: 159591910
Test: cd external/rust; mma; m crosvm.experimental
Change-Id: Ia1124e09f48cd05e39f094bbcb988622ebd2272f