Fix Haskell coverage results

There are two current issues with the coverage values:

- we don't import all modules, thus leading to incomplete
  coverage results (too optimistic);
- we use hpc in its default mode (intersection), which means that even
  modules which do have coverage results but are not used in all
  binaries we test will be dropped from the results; thanks to Agata,
  passing --union to hpc is enough to have better results (don't
  remember why this wasn't there in the first place…)

After adding more modules to the import list and fixing the combining
mode, we now have a complete list of modules in coverage results, many
with zero coverage, so our overall coverage has dropped to about 60%.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>