rust: Include Soong module name as metadata

If we want to build a component which uses two versions of a crate in
the dependency graph (for example, libc in std and libc from
external/crates), Rust requires at least one of them have a metadata
field to differentiate them.

Since Soong module names should be distinct for a given target, they
should serve as metadata.

Bug: 147397356
Test: Build a rust library with "rlibs: ["liblibc"]" in the blueprint
Change-Id: I5d85ea8b3197972c09de301c9523efaff08adf81