add data-dir to InstalledPackageInfo

This adds the `data-dir` to `InstalledPackageInfo` so that it's available to GHC. It still needs a few changes in `ghc` and `ghc-pkg` to actually be used.

This is required for GHCJS to be able to correctly make `data-files` for packages available at link time, without having to expose all contents of the hard drive over HTTP. See https://ghc.haskell.org/trac/ghc/ticket/9808

I'm submitting this part first since this is the only part of the GHCJS support patch that requires changes in GHC (`bin-package-db` serialization). I'll submit those once this has been merged.