profiles: Produce a top-level Info 'dir' file.

Fixes <http://bugs.gnu.org/18305>.
Reported by Brandon Invergo <brandon@gnu.org>.

* guix/profiles.scm (manifest-inputs, info-dir-file): New procedures.
  (profile-derivation): Use them.  Add #:info-dir? parameter and honor
  it.
* guix/scripts/package.scm (guix-package): Call 'profile-derivation'
  with #:info-dir? #f when the 'bootstrap? option is set.
* tests/profiles.scm ("profile-derivation"): Pass #:info-dir? #f.