gexp: 'local-file' properly resolves non-literal relative file names.

* guix/gexp.scm (local-file): Distinguish the case where FILE is a
literal string and when it's not.  Add a clause for when FILE is not a
literal string.
* tests/gexp.scm ("local-file, non-literal relative file name"): New test.
* doc/guix.texi (G-Expressions): Update accordingly.