edit: Use 'specification->location' to read information from the cache.

edit: Use 'specification->location' to read information from the cache.

That way 'guix edit' doesn't need to load any package module.

* gnu/packages.scm (find-package-locations, specification->location):
New procedures.
* guix/scripts/edit.scm (package->location-specification): Rename to...
(location->location-specification): ... this.  Expect a location object
instead of a package.
(guix-edit): Use 'specification->location' instead of
'specification->package'.
* tests/packages.scm ("find-package-locations")
("find-package-locations with cache")
("specification->location"): New tests.