environment: Improve error reporting for '-l'.

This fixes two problems:

  1. Load errors would print an ugly backtrace.
  2. When FILE was an absolute name, 'load' was passed an incorrect file
     name.

* guix/scripts/environment.scm (options/resolve-packages): Use 'load*' instead
  of 'load'.

guix/scripts/environment.scm

@@ -200,7 +200,7 @@ (define (options/resolve-packages opts)
                          (package-outputs package)))))
                 (('load . file)
                  ;; Add all the outputs of the package defined in FILE.
-                 (let ((package (load (string-append (getcwd) "/" file))))
+                 (let ((package (load* file (make-user-module '()))))
                    (map (lambda (output)
                           `(package ,package ,output))
                         (package-outputs package))))