When passing a package database to ghc-pkg via GHC_PACKAGE_PATH or --package-db, ${pkgroot} does not get computed properly if the input path contains a trailing slash.

Default behavior:

$ ghc-pkg describe base | grep pkgroot

    pkgroot: "/usr/lib/ghc-7.10.2"

Correct behavior (no trailing slash):
    $ ghc-pkg --package-db /usr/lib/ghc-7.10.2/package.conf.d describe base | grep pkgroot
    pkgroot: "/usr/lib/ghc-7.10.2"

    $ GHC_PACKAGE_PATH=/usr/lib/ghc-7.10.2/package.conf.d ghc-pkg describe base | grep pkgroot
    pkgroot: "/usr/lib/ghc-7.10.2"

Incorrect behavior (with trailing slash):
    $ ghc-pkg --package-db /usr/lib/ghc-7.10.2/package.conf.d/ describe base | grep pkgroot
    pkgroot: "/usr/lib/ghc-7.10.2/package.conf.d"

    $ GHC_PACKAGE_PATH=/usr/lib/ghc-7.10.2/package.conf.d/ ghc-pkg describe base | grep pkgroot
    pkgroot: "/usr/lib/ghc-7.10.2/package.conf.d"

When this bug happens, ghc-pkg check complains about missing files for packages using ${pkgroot}.

This bug happens because ${pkgroot} is computed using takeDirectory. It should instead use (takeDirectory . dropTrailingPathSeparator)