Support Haskell stack by making HaskellRequirement
Brings support for Haskell stack by removing CabalRequirement for HaskellRequirement, which can handle multiple package managers. Similar to DistributionRequirement, with a few changes:
- Since packages are pulled from hackage, cabal and stack will always use the same package name, managers are specified via args
- I have added
get_installed_version
(previously not implemented in CabalRequirement), for Cabal since multiple versions can be installed, it returns the first(prioritised by visiblity) version.
Closes #67 (closed)