refactor: caching logic (#90)

Currently we are not keeping cache entries for files which were out of
scope of compiler (might be dirty or not)

Because of that, when running `forge compile`, then running `forge
compile --skip ...` which will filter some artifacts out, and then
running `forge compile` again, some artifacts will be recompiled,
because `--skip` run removed those entries from cache.

Solution is to keep those entries in cache (we already silently keep
artifacts anyway).