Changes for relative html urls, part II

* hptool/src/HaddockMaster.hs
  * Call the url munging OS-hooks higher up (right before
    using the parameters in the haddock command line), to
    avoid passing too much extra info down
  * Rename the readArgs function to haddockReadArg, and
    export it

* hptool/src/OS.hs
* hptool/src/OS/Internal.hs
* hptool/src/OS/Mac.hs
* hptool/src/OS/Win.hs
  * Remove PkgInstallDirs in favor of
    osPackageConfigureExtraArgs to add the needed args for
    any OS
  * Change the url mungers, osGhcPkgPathMunge and
    osPlatformPkgPathMunge, to work on HaddockPkgLoc rather
    than just the FilePath

* hptool/src/Target.hs
  * Follow the changes from HaddockMaster
  * use osPackageConfigureExtraArgs rather than setting any
    paths
* hptool/src/Types.hs
  * Add HaddockPkgLoc for the pair of strings representing
    the info retrieved from the package conf files, used for
    making the relative url links; (putting this into Types
    avoids any circular imports between the OS layer and
    HaddockMaster)