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.