* guix/ui.scm (read/eval): New procedure.
(read/eval-package-expression): Use it.
* guix/scripts/build.scm (derivations-from-package-expressions): Rename to...
(derivation-from-expression): ... this. Accept procedures, under the
assumption that they are monadic thunk.
(show-help): Adjust accordingly.
(guix-build): Ditto.
* tests/guix-build.sh: Add test.
* doc/guix.texi (Invoking guix build): Augment description of '-e'.