import: print: Honor the outputs of inputs (!).

import: print: Honor the outputs of inputs (!).

Fixes <http://bugs.gnu.org/35893>.
Reported by Jesse Gibbons <jgibbons2357@gmail.com>.

* guix/import/print.scm (package->code)[package-lists->code]: Preserve
OUT in the result.
* tests/print.scm (define-with-source): New macro.
(pkg): Use it.
(pkg-source): New variable.
(pkg-with-inputs, pkg-with-inputs-source): New variables.
("simple package"): Refer to 'pkg-source'.
("package with inputs"): New test.