Support cabal.projects

[sevanspowell]

• Allow user to specify "package directories" corresponding to the
  source directories of packages in a cabal.project.
• Add functionality required to allow the user to specify the hpc data
  search directories explicitly, rather than searching for it in the
  usual places. This is primarily motivated by the Nix package
  manager, where hpc output is usally written to some folder outside
  of the current directory (e.g. to
  /nix/store/HASH-my-lib-0.1.0.0/share/hpc) as well as cabal.projects,
  where the hpc output directory might be
  "dist-newstyle/build/$platform/$compiler/$package/hpc". Multiple hpc
  directories can be specified, for e.g., one for each package in a
  cabal.project.
• The filepath listed in a mix file doesn't include the sub-directory
  of the package in a cabal.project. So for cabal.projects, the
  filepath used as the index in the TestSuiteCoverageData is now
  prefixed with the sub-directory of the package. The behaviour when
  not using the "--package-dir" argument, or when using "--package-dir
  ./" is unchanged.
• Gave a Monoid instance to TestSuiteCoverageData so we can easily use
  folds to sum the data up.
• Separated the coveralls.io specific logic and the coverage data
  logic. The coverage data could be re-used for other coveralls-style
  tools.
• Source files are now excluded before searching for them, so that if
  the file does not exist, it does not throw an error (primarily
  motivated by out-of-source builds such as Nix, where modules such as
  Path_library.hs can't be found in the build directory).
• Changed the structure of getCoverageData a little, and added some
  commentary, so each of the steps required to read all the coverage
  data are a bit clearer.

[sevanspowell]

cc @guillaume-nargeot

[JasonSome]

This repository seems dead years now... I cannot even make it work with the new Cabal 3.0.0 and GHC >= 8.8.

[sevanspowell]

@JasonSome Yep, I've figured as much.

As for working with newer versions of Cabal, this might help: sevanspowell@3193755