When generating a haddock .t file for a library, don't save it in the
`dist-install/doc` directory for that library, as then it gets copied to
the installation directory during `make install` by `ghc-cabal copy`.
Instead, save it a few directories up; putting it next to
`haddock-prologue.txt` seemed appropriate.

Test Plan: run `make` in `tests/perf/haddock`.

Differential Revision: https://phabricator.haskell.org/D903