Learn about -package

If you use a package that ghc doesn't guess about, be sure to list it in
`GHC_PACKAGES`. The issue with `System.FilePath` was around the missing
`filepath` package.

See `ghc-pkg list` for a list of packages, and
http://www.haskell.org/ghc/docs/7.0.4/html/libraries/index.html for the
list of packages that come with ghc.