HPC artifacts are written and read from pkg-db

This commit re-designs the mechanism by which we make the .mix files of
libraries available to produce the Haskell Program Coverage report after
running testsuites.

The idea, for the Cabal library, is:

* Cabal builds libraries with -fhpc, and store the hpc artifacts in
  build </> `extraCompilationArtifacts`
* At Cabal install time, `extraCompilationArtifacts` is copied into the
  package database
* At Cabal configure time, we both
    - receive as --coverage-for flags unit-ids of library components
      from the same package (ultimately, when #9493 is resolved, we will
      receive unit ids of libraries in other packages in the same
      project too),
    - and, when configuring a whole package instead of just a testsuite
      component, we determine the unit-ids of libraries in the package
  these unit-ids are written into `configCoverageFor` in `ConfigFlags`
* At Cabal test time, for each library to cover (stored in
  `configCoverageFor`), we look in the package database for the hpc
  dirs, which we eventually pass along to the `hpc markup` call as
  `--hpcdir` flags

As for cabal-install:

* After a plan has been elaborated, we select the packages which can be
  covered and pass them to Cabal's ./Setup configure as
  --coverage-for=<unit-id> flags.
    - Notably, valid libraries are non-indefinite and
      non-instantiations, since HPC does not support backpack.
    - Furthermore, we only include libraries in the same package as the
      component being configured, despite possibly there being
      more library components in other packages of the same project.
      When #9493 is resolved, we could lift this restriction and pass
      all libraries local to the package as --coverage-for. See
      `determineCoverageFor` and `shouldCoverPkg` in Distribution.Client.ProjectPlanning.

Detail:
    We no longer pass the path to the testsuite's mix dirs to `hpc
    markup` because we only ever include modules in libraries, which
    means they were previously unused.

Fixes #6440 (internal libs coverage), #6397 (backpack breaks coverage),
doesn't yet fix #8609 (multi-package coverage report) which is tracked
in #9493, and fixes in a new way the previously fixed #4798, #5213.