packages: Add 'file-type' field to 'search-path-specification'.

Fixes <http://bugs.gnu.org/18033>.

* guix/packages.scm (<search-path-specification>): Rename 'directories'
  field to 'files'.  Add 'file-type'.
  (search-path-specification->sexp): Honor 'file-type'.
* gnu/packages/autotools.scm, gnu/packages/bootstrap.scm,
  gnu/packages/cross-base.scm, gnu/packages/games.scm,
  gnu/packages/gcc.scm, gnu/packages/glib.scm,
  gnu/packages/guile.scm, gnu/packages/man.scm,
  gnu/packages/perl.scm, gnu/packages/pkg-config.scm,
  gnu/packages/python.scm, gnu/packages/ruby.scm,
  gnu/packages/xfce.scm: Change 'directories' to 'files'.
* tests/packages.scm ("search paths"): Change 'directories' field to
  'files'.
* guix/scripts/environment.scm (for-each-search-path): Likewise.