gnu: ghc: Add GHC_PACKAGE_PATH native search path.

gnu: ghc: Add GHC_PACKAGE_PATH native search path.

Benefits include: 'guix environment' more useful for ghc libraries, more
useful 'guix package --search-paths' for installed ghc libraries, cleaner
package recipes: no longer need to propagate runtime package dependencies.

* guix/build/haskell-build-system.scm (configure): Unset GHC_PACKAGE_PATH
  around cabal configure.
  (make-ghc-package-database): Use pattern directory search.
  (register): Install complete package database for the current package.
* gnu/packages/haskell.scm (ghc): Add native-search-paths field.