packages: 'package-field-location' preserves the original file name.

Fixes <https://bugs.gnu.org/39425>.
Reported by Alex ter Weele <alex.ter.weele@gmail.com>.

* guix/packages.scm (package-field-location): Remove 'with-fluids' for
'%file-port-name-canonicalization'.  Change the 'file' field of the
resulting location to FILE.