rust: validate existence of library source

While rust_bindgen modules may be interpreted as libraries, they do not
have a reference to the generated source until bindgen has been
executed. For now, ignore these modules.

Update the unit tests to cover the rust_bindgen case.

Test: SOONG_GEN_RUST_PROJECT=1 m nothing
Bug: 162881856
Change-Id: I329d0fe3d94b77d395c3684f55ab01544ff7c18f