guix build: Support '--with-source' along with '-e'.

* guix/scripts/build.scm (derivation-from-expression): Remove.
  (options->derivations): Handle pairs of the form
  "('argument . (? derivation?))".
  (options/resolve-packages): Add 'store' parameter; update caller.  Add
  'system' variable.  Add case for 'expression pairs.
* guix/scripts/archive.scm (derivation-from-expression): New procedure.