* This adds more logic for the cabalProjectToNix idf
That is, something like this:
```
nix-build --expr 'with import ./. {}; callCabalProjectToNix { index-state = "2019-04-30T00:00:00Z"; src = /some/path; }'
```
should produce something that can be build with mkCabalProjectPkgSet.
* Make sure the hackageTarball's store path doesn't change
previously the fetchurl would produce a different store path each
and every time as hackage's index is a moving target. With this
impurity setup, we can ignore this.
* Fix test
* Proper name
* Re-enable test
* Allow to parameterize over the `system` for the test default.nix
* Copy instead of link for ifds.
This makes me really sad.
