#1945 Make hpc-related activity conditional on --enable-library-coverage flag

Instead of applying hpc flags directly via 'enableCoverage' I've added a 'withCoverage' flag to LocalBuildInfo that is then checked before setting HPXTIXFILE or calling 'markupTest' or 'markupPackage' and used to generate -fhpc and -hpcdir flags in the GHC-specific code.

Added initially-failing now passing tests to PackageTests/TestSuiteExeV10.
This module already exported `htmlDir` and `tixDir`, `mixDir` is just consistent (and already existed I just exposed it). Personally I think the name makes sense within the context of the Hpc module though I'm happy to change it to anything you like. Would qualifying its use in this module suffice perhaps? i.e.

    import qualified Distribution.Simple.Hpc as Hpc ( mixDir )
    ...
        withCoverage = if fromFlag $ configLibCoverage cfg
                         then Just $ Hpc.mixDir distPref (display $ package pkg_descr')
                         else Nothing