#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.
I think qualified imports can often help readability. Note that you don't need the `( mixDir )` part if you use qualified imports (one of the benefits of using them).