Partly fixes <http://bugs.gnu.org/18033>.
* guix/build/utils.scm (search-path-as-list): Rename 'sub-directories'
parameter to 'files'. Add #:type parameter and honor it.
(set-path-environment-variable): Likewise. Pass #:type to
'search-path-as-list'.
* guix/packages.scm (search-path-specification->sexp): Add 'directory as
the last item of the tuple.
* guix/build/gnu-build-system.scm (set-paths): Add 'type' to search-path
pattern. Pass #:type to 'set-path-environment-variable'.
6aa47e3883