guix package: Allow removal of a specific package output.

Fixes <http://bugs.gnu.org/15756>.

* guix/profiles.scm (<manifest-pattern>): New record type.
  (remove-manifest-entry): Remove.
  (entry-predicate, manifest-matching-entries): New procedures.
  (manifest-remove): Accept a list of <manifest-pattern>.
  (manifest-installed?): Replace 'name' parameter by 'pattern', a
  <manifest-pattern>.
* guix/scripts/package.scm (options->removable): Return a list of
  <manifest-pattern>.
  (guix-package)[process-action]: Use 'manifest-matching-entries' to
  compute the list of packages to remove.
* tests/profiles.scm: New file.
* Makefile.am (SCM_TESTS): Add it.

Add (guix profiles).

* guix/scripts/package.scm (show-what-to-remove/install): New procedure,
  moved from...
  (guix-package): ... here.
  (<manifest>, make-manifest, <manifest-entry>,
  profile-manifest, manifest->sexp, sexp->manifest, read-manifest,
  write-manifest, remove-manifest-entry, manifest-remove,
  manifest-installed?, manifest=?, profile-regexp, generation-numbers,
  previous-generation-number, profile-derivation, generation-number,
  generation-file-name, generation-time, lower-input): Move to...
* guix/profiles.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.