Better defaults (doExactConfig = true; doHaddock = false; non-reinstallable lib:ghc)

[angerman]

We should change the defaults:

• doExactConfig = true for all stack or cabal plans. We can rely on those tools to have computed the expected build plan and should not start questioning that one in haskell.nix. This may incur a tiny performance hit on CI as we need to compute lots of constraints and pass them to SETUP_HS, but should make the end user experience much more likely to succeed.
• doHaddock = false, if a package is misconfigured or lacks haddocks, haddock will fail and the package will fail to build. This is an unnecessary annoyance. We should rather default to no haddocks and have them enabled on a per-need basis. Also building haddocks requires an additional step that costs time and is not essential to the build on CI.
• We currently try to always use the reinstall able lib:ghc, this not only means we need to maintain that custom package (as well as other boot packages) ourselves, but also that this fails with plugins and doctest and other tools that are loaded or load ghc(i), as we end up with duplicate lib:ghc's. This is fragile, thus we should disable it by default, but allow it opt-in.