Avoid Cargo's caching if any lint crates changed

[xFrednet]

Cargo includes an automatic caching mechanism, that avoids running the rustc diver if nothing changed. This mechanism doesn't account for potential changes inside the loaded lint crates.

AFAIK, it's possible to add more files to the cache check. If we add the compiled lint crates, it might just work itself out :)

This bug was extracted from: #189

Feedback loop

I had to run cargo clean each time I change the lint and want to check how it works. My feedback loop was:

1. Change the custom lint's code.
2. Run cargo clean
3. Run cargo marker

If I omit the step 2 I still see the output from the old version of my lint code.

I remember there was a different problem in cargo-clippy a long ago, which was also worked around with cargo clean to bust the incremental compilation cache (rust-lang/rust-clippy#2604 (comment)). It isn't what happens here, but I hope marker won't have the same caveat.