guix lint: Remove "guix lint: " prefix from warnings.

This allows editors to parse warnings correctly.

* guix/scripts/lint.scm (emit-warning): Use 'format' instead of
  'warning', to avoid the "guix lint: " prefix in messages.
* tests/lint.scm (call-with-warnings): Indent.

guix/scripts/lint.scm

@@ -81,10 +81,10 @@ Run a set of checkers on the specified package; if none is specified, run the ch
   ;; provided MESSAGE.
   (let ((loc (or (package-field-location package field)
                  (package-location package))))
-    (warning (_ "~a: ~a: ~a~%")
-             (location->string loc)
-             (package-full-name package)
-             message)))
+    (format (guix-warning-port) (_ "~a: ~a: ~a~%")
+            (location->string loc)
+            (package-full-name package)
+            message)))
 
 
 ;;;

tests/lint.scm

@@ -39,10 +39,10 @@
            (home-page #f) (license #f) ))
 
 (define (call-with-warnings thunk)
-       (let ((port (open-output-string)))
-         (parameterize ((guix-warning-port port))
-           (thunk))
-         (get-output-string port)))
+  (let ((port (open-output-string)))
+    (parameterize ((guix-warning-port port))
+      (thunk))
+    (get-output-string port)))
 
 (test-assert "synopsis: ends with a period"
   (->bool