`runghc Setup.hs haddock` does not allow for `--haddock-options=--prologue=my-prologue`

Haddock only allows `--prologue` to be specified once, ending in failure if the option is specified more than once.

`runghc Setup.hs haddock` makes use of the `--prologue` option itself, setting it to the content of the Cabal file's `description` field.

However, as a consequence, a user then has no mechanism to specify his or her own Haddock prologue.

A possible solution:

Cabal (the library) provides a `--haddock-prologue=` option that, if specified, prevails over Cabal's default use of `--prologue`.

Related Stack issue:
* https://github.com/commercialhaskell/stack/issues/6718

Start by reproducing the behavior with `runghc Setup.hs haddock` and `--haddock-options=--prologue=my-prologue`. Read the Cabal library's Haddock option handling and related tests, then verify that the proposed `--haddock-prologue=` option lets users provide a prologue without Haddock receiving duplicate `--prologue` options.

Written by the indexing model from the issue text.