Support coverage results for non-library code

See #1008 . I'm thinking this would look like adding a `--coverage-exe` flag. Or, some other name, TBH one of the reasons this doesn't exist is that there's no concise name for this flag that obviously corresponds to its behavior.

The implementation would look like:
- Have a variant of the tix file munging which filters out all the package-qualified module names. Use this to process the resulting tix file.
- Only provide one `--srcdir` argument to the hpc program - the path to the package associated with the test-suite.
- Omit the `--include` argument.

Due to some [hpc funkiness](https://ghc.haskell.org/trac/ghc/ticket/1853), these results will not be able to be included in the unified coverage report, or any coverage reports that span multiple packages.