'cabal get PACKAGE' is the new name of 'cabal unpack'.
'cabal get --source-repository' reads the source-repositories from a package's
description, determines which VCS to use, and then creates a local repository or
branch of the package's repository.
'cabal get --source-repository=[head|this|...]' additionally allows to specify
which source-repository to use.
Based on the code originally written by John Millikin <firstname.lastname@example.org>.