Move more checks into checkPhase

[KaiserKarel]

Right now we are defining many top-level checks, which is quite noisy. Derivations export a checkPhase, which means that for each derivation, we could verify the integrity of that output without cluttering the global namespace.

[KaiserKarel]

Decided on for now holding off, as we'd like to quickly observe failing tests in CI. Probably around 100 checks we ought to switch. A nice pattern is pulling in the checks in the checkPhase to force evaluation:

unionvisor = crane.stable.buildPackage (commonArgs // { 
        doCheck = true;
        checkPhase = ''
          ls -a ${checks.unionvisor-clippy}
        '';
      });

[benluelo]

related: #525