utils: Add 'switch-symlinks', moved from (guix ui).

* guix/ui.scm (switch-symlinks): Move to...
* guix/utils.scm: ... here.  New procedure.
* guix/scripts/pull.scm: Use it.

guix/scripts/pull.scm

@@ -18,6 +18,7 @@
 
 (define-module (guix scripts pull)
   #:use-module (guix ui)
+  #:use-module (guix utils)
   #:use-module (guix scripts)
   #:use-module (guix store)
   #:use-module (guix config)

guix/ui.scm

@@ -73,7 +73,6 @@ (define-module (guix ui)
             read/eval
             read/eval-package-expression
             location->string
-            switch-symlinks
             config-directory
             fill-paragraph
             texi->plain-text
@@ -715,13 +714,6 @@ (define (location->string loc)
     (($ <location> file line column)
      (format #f "~a:~a:~a" file line column))))
 
-(define (switch-symlinks link target)
-  "Atomically switch LINK, a symbolic link, to point to TARGET.  Works
-both when LINK already exists and when it does not."
-  (let ((pivot (string-append link ".new")))
-    (symlink target pivot)
-    (rename-file pivot link)))
-
 (define (config-directory)
   "Return the name of the configuration directory, after making sure that it
 exists.  Honor the XDG specs,

guix/utils.scm

@@ -74,6 +74,7 @@ (define-module (guix utils)
             arguments-from-environment-variable
             file-extension
             file-sans-extension
+            switch-symlinks
             call-with-temporary-output-file
             call-with-temporary-directory
             with-atomic-file-output
@@ -557,6 +558,13 @@ (define (file-sans-extension file)
         (substring file 0 dot)
         file)))
 
+(define (switch-symlinks link target)
+  "Atomically switch LINK, a symbolic link, to point to TARGET.  Works
+both when LINK already exists and when it does not."
+  (let ((pivot (string-append link ".new")))
+    (symlink target pivot)
+    (rename-file pivot link)))
+
 (define* (string-replace-substring str substr replacement
                                    #:optional
                                    (start 0)