build-system/haskell: Update configure flags, 'haddock' and %standard-phases.

* guix/build/haskell-build-system.scm (%standard-phases): move 'haddock phase
  before 'install phase.
* guix/build/haskell-build-system.scm (haddock): Simplify it as the 'install
  phase takes care of copying files.
* guix/build/haskell-build-system.scm (configure): Add '--libsubdir' flag.
  Fix use of '--extra-include-dirs' and '--extra-lib-dirs' flags.  Use 'doc',
  'bin' and 'lib' outputs if they are defined.
* guix/build/haskell-build-system.scm (make-ghc-package-database, register):
  Aligh location of 'package.conf.d' directory with '--libsubdir' flag.