Historically ghc implemented relocatable packages by allowing
"$topdir" in the package registration info and having ghc expand
this with its notion of $topdir. The topdir refers to where ghc
itself is installed (specifically the libdir).

The ${pkgroot} spec takes this idea and makes it portable.
(http://www.haskell.org/pipermail/libraries/2009-May/011772.html)
Instead of paths relative to where ghc is installed, they can be
relative to the package database itself. Thus it is no longer a
ghc-specific idea and can work for package collections other than
the global package db.