Changed handling of nodes in src/parser/grammar.lisp

Since the `import' rule returns a list of nodes - the toplevel nodes
in the parsed file - the other toplevel rules also have to return
lists of nodes. The `proto' rule concatenates these lists.

* src/parser/grammar.lisp (defrule/locations package): return a
  one-element list of produced nodes
  (defrule packaged-element): likewise
  (defrule/locations proto): concatenate lists of nodes