Notice if a run is going to be a no-op before generating rustdoc JSON

[obi1kenobi]

From a recent cargo semver-checks && cargo publish run on a new release of cargo-semver-checks itself:

     Parsing cargo-semver-checks v0.30.0 (current)
      Parsed [  43.575s] (current)
     Parsing cargo-semver-checks v0.29.1 (baseline)
      Parsed [   8.502s] (baseline)
    Checking cargo-semver-checks v0.29.1 -> v0.30.0 (major change)
     Checked [   0.000s] 0 checks; 0 passed, 62 unnecessary
    Finished [  52.092s] cargo-semver-checks

We spent 52s generating rustdoc JSON that we didn't even look at, because no lints were scheduled to run as this is a major bump.

Perhaps we can handle this as part of our refactor in #599.