download: Export 'maybe-expand-mirrors'.

* guix/build/download.scm (uri-vicinity, maybe-expand-mirrors): New
  procedures.
  (url-fetch): Remove them from here.

guix/build/download.scm

@@ -29,6 +29,7 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
   #:export (open-connection-for-uri
+            maybe-expand-mirrors
             url-fetch
             progress-proc
             uri-abbreviation))
@@ -279,17 +280,15 @@ which is not available during bootstrap."
     (lambda (key . args)
       (print-exception (current-error-port) #f key args))))
 
-(define* (url-fetch url file #:key (mirrors '()))
-  "Fetch FILE from URL; URL may be either a single string, or a list of
-string denoting alternate URLs for FILE.  Return #f on failure, and FILE
-on success."
-  (define (uri-vicinity dir file)
-    ;; Concatenate DIR, slash, and FILE, keeping only one slash in between.
-    ;; This is required by some HTTP servers.
+(define (uri-vicinity dir file)
+  "Concatenate DIR, slash, and FILE, keeping only one slash in between.
+This is required by some HTTP servers."
   (string-append (string-trim-right dir #\/) "/"
                  (string-trim file #\/)))
 
-  (define (maybe-expand-mirrors uri)
+(define (maybe-expand-mirrors uri mirrors)
+  "If URI uses the 'mirror' scheme, expand it according to the MIRRORS alist.
+Return a list of URIs."
   (case (uri-scheme uri)
     ((mirror)
      (let ((kind (string->symbol (uri-host uri)))
@@ -303,8 +302,12 @@ on success."
     (else
      (list uri))))
 
+(define* (url-fetch url file #:key (mirrors '()))
+  "Fetch FILE from URL; URL may be either a single string, or a list of
+string denoting alternate URLs for FILE.  Return #f on failure, and FILE
+on success."
   (define uri
-    (append-map maybe-expand-mirrors
+    (append-map (cut maybe-expand-mirrors <> mirrors)
                 (match url
                   ((_ ...) (map string->uri url))
                   (_       (list (string->uri url))))))