Now the tarballs are downloaded only once but now each package name and
version is unpacked independently in its own directory. Then patches are
applied there.
Doesn't quite work because srcDir is unique by url, so if you have the
same url with different versions, things will collide. This is a bug
that might affect patching too.
I was applying patches only when making the source distribution but
patches might modify the cabal file which goes into the index (yes one
should use revisions for that).
I am now applying patches right after I have the package source.
1. Foliage takes as input a complete description of the index, where
source distributions and revisions come with a timestamp. This allows
us to recreate the entire index in a reproducible way.
2. Added a experimental command to import an index from a Hackage (as
downloaded with Cabal). This was originally a testing/development
need but there might be different use cases.
