`OptVersionReq::Locked` stores the original version requirement and the
acutal locked version. This makes error message describing more precise.

Previously we use exact semver version req to synthesize locked version
req. Now we make those need a truly locked to be `OptVersionReq::Locked`.

When encounter resolver error with locked dependency, we now show
original version req along with the exact locked version.

Instead of creating another VersionReq, this can reduce some extra
efforts on version matching logic. Though it also may introduces
divergence from semver's matching logic. Accordingly, we borrow unit
tests from semver crate and compare the matching results to prevent
future breaks.