graph: Allow store file names for 'derivation' and 'references' graphs.

* guix/scripts/graph.scm (%derivation-node-type)[convert]: Add
'derivation-path?' and catch-all clauses.
(%reference-node-type)[convert]: Add 'store-path?' and catch-all
clauses.
(assert-package, nodes-from-package): New procedures.
(%package-node-type, %bag-node-type,%bag-with-origins-node-type)
(%bag-emerged-node-type): Add 'convert' field
(guix-graph): Rename 'packages' to 'items' and
allow 'store-path?' arguments.
* guix/graph.scm (<node-type>)[convert]: Adjust comment.
* doc/guix.texi (Invoking guix graph): Document it.