rust: Don't append '.vendor' to vendor modules.

Rust vendor-only modules would have the '.vendor' subname appended to
them, which meant that 'm <vendor_module>' would not work -- instead
you would need to call 'm <vendor_module>.vendor', which leads to some
confusion.

This CL fixes the behavior by using the same SubName logic as the cc
module.

Bug: 205577906
Test: m <vendor_module> # works without .vendor suffix
Change-Id: I6ba18ce1d7281a1f8342ed6014644b48009d78e0