This improves on the previous warning hack to keep a cache of parsed
GenericPackageDescriptions, and avoid rerunning hpack.

There are some TODOs added in this commit. One further point of concern:
should we opt-out of caching the results of parsing index files? I'm
imagining that when loading a snapshot, this may result in a lot of
memory usage. (Then again, this may already be the case, see #3586.)

This is a large change that fell out from trying to clean up the mess
left from the previous commit. The result here should be signficant
simplification of the code paths around parsing cabal files. In fact,
there are a few existing TODOs that got hit by this.

Instead: just cache the results of cabal file parsing, and run hpack
when doing so. This (as the previous few patches) involved much more
overhaul than seems like it should. The best way to do this reliably is
to only expose a single function from Stack.Package which can run hpack.
In turn, this ended up requiring a conversion of a bunch of parts of the
code base from passing around Path Abs File (pointing to the cabal file
itself) to instead pass around Path Abs Dir (pointing to the directory).

I think this is a good change, once against simplifying things a bit
more.

See #3616