rust: Mutate prebuilt modules dylib/rlib

This change makes it possible to use a single module to provide both
dylib and rlib varieties of a library. This allows the use of libstd and
libtest from a rustlibs property, allowing linkage type to change
for different variants.

Bug: 159718669
Test: cd external crates; mma; m crosvm.experimental
Change-Id: I477c4d2faec63703fdc6dd42ba020747d6a50714