packages: Move grafting parameter to (guix derivations).

* guix/packages.scm (%graft?, set-grafting): Move to...
* guix/derivations.scm: ... here.

guix/derivations.scm

@@ -97,6 +97,9 @@ (define-module (guix derivations)
             build-derivations
             built-derivations
 
+            %graft?
+            set-grafting
+
             build-expression->derivation)
 
   ;; Re-export it from here for backward compatibility.
@@ -1287,3 +1290,16 @@ (define %build-inputs
 
 (define built-derivations
   (store-lift build-derivations))
+
+;; The following might feel more at home in (guix packages) but since (guix
+;; gexp), which is a lower level, needs them, we put them here.
+
+(define %graft?
+  ;; Whether to honor package grafts by default.
+  (make-parameter #t))
+
+(define (set-grafting enable?)
+  "This monadic procedure enables grafting when ENABLE? is true, and disables
+it otherwise.  It returns the previous setting."
+  (lambda (store)
+    (values (%graft? enable?) store)))

guix/packages.scm

@@ -103,7 +103,6 @@ (define-module (guix packages)
             &package-cross-build-system-error
             package-cross-build-system-error?
 
-            %graft?
             package->bag
             bag->derivation
             bag-transitive-inputs
@@ -112,9 +111,7 @@ (define-module (guix packages)
             bag-transitive-target-inputs
 
             default-guile
-
             set-guile-for-build
-            set-grafting
             package-file
             package->derivation
             package->cross-derivation
@@ -678,10 +675,6 @@ (define derivation
                         (package package)
                         (input   x)))))))
 
-(define %graft?
-  ;; Whether to honor package grafts by default.
-  (make-parameter #t))
-
 (define* (package->bag package #:optional
                        (system (%current-system))
                        (target (%current-target-system))
@@ -918,12 +911,6 @@ (define (set-guile-for-build guile)
     (let ((guile (package-derivation store guile)))
       (values (%guile-for-build guile) store))))
 
-(define (set-grafting enable?)
-  "This monadic procedure enables grafting when ENABLE? is true, and disables
-it otherwise.  It returns the previous setting."
-  (lambda (store)
-    (values (%graft? enable?) store)))
-
 (define* (package-file package
                        #:optional file
                        #:key