Replace `rustc_semver` with `rustc_session::RustcVersion` Allows dropping a dependency but there is a behaviour change here, the following versions are no longer accepted: * `1` (would need to be `1.0` or `1.0.0`) * `1.0.0-alpha` * `1.0.0-alpha.2` * `1.0.0-beta` But this seems unlikely to effect anybody, I did some GitHub searching and found no occurrences outside our UI tests r? `@flip1995` changelog: none