Correctly provision build tools in all situations

This patch ensures that we correctly provision executables declared
in the build-tool-depends fields in all circumstances:

  - whether the build tool is external (from another package) or
    internal (declared in the current package)
  - whether the build tool is used at compile time (e.g. in a pre-build
    rule or in a Template Haskell splice) or at run time (e.g. when
    running a test-suite, benchmark or executable).

Note that correctly provisioning a build tool requires two pieces of
information:

  - making it available in PATH,
  - ensuring it has the correct environment variables overrides;
    in particular, the build tool needs to be able to find its own
    data directory.

The test case BuildToolPaths checks all of these situations are handled
correctly.