gexp: 'program-file' has a new #:module-path parameter.

* guix/gexp.scm (<program-file>): Add 'path' field.
(program-file): Add #:module-path parameter and honor it.
(program-file-compiler): Honor the 'path' field.
* tests/gexp.scm ("program-file #:module-path"): New test.
* doc/guix.texi (G-Expressions): Update.