When searching the RUST_PATH, rustpkg ignores versions

[catamorphism]

In an upcoming pull request, rustpkg will gain the ability to find installed libraries in the RUST_PATH without necessarily rebuilding them. However, it will ignore the requested version (if there is one) when searching for libraries. This is wrong; if you request foo#0.3, it should ignore an installed foo-0.4, for example.

[catamorphism]

Blocked on #9193

[catamorphism]

Fixed pending try build.

[gereeter]

Rustpkg is gone, so this should probably be closed.