* Add support for external Hackage repositories
Currently haskell.nix is not able to build Cabal projects that depend on
packages from private Hackage repositories, as it make only main Hackage
available to cabal. This is unfortunate.
This commit adds this functionality, by allowing the user to pass
`extra-hackages` and `extra-hackage-tarballs` to `mkPkgSet` and
`callCabalToNix` respectively, to add as much extra repositories as
needed.
This repositories are first made available to Cabal when calling
`v2-configure`, resulting in correct plans. Later they are combined with
global Hackage when building dependencies of the local packages.
* Use cabal.project.freeze if available
Currently callCabalProjectToNix does not copy `cabal.project.freeze`
from source directory, leading to different build plans when building
components with nix and when building project with `cabal new-build`
inside `nix-shell`.
This behavior is undesired, so this commits fixes it.
* Add tests for extra-hackages functionality
Co-authored-by: Moritz Angermann <moritz.angermann@gmail.com>
