Support caching based on --PROG-options

Currently changing --ghc-options leads to recompilation of the affected packages.
Even if these have been compiled with the same flags before.

In other words the cached packages are overwritten by the ones with the new flags.

It's likely not a good idea cache these by default as it can easily lead to very large caches.
But it would be incredibly helpful as optional feature for things like benchmarks comparing different GHC flags. Currently I use different store dirs just to avoid triggering the recompilation when rerunning benchmarks.