import/cran: Restore format-inputs.

This fixes a regression introduced in commit
e6223017d9 causing the selected input style to
be ignored.

* guix/import/cran.scm (format-inputs): New procedure.
(maybe-inputs): Use it.

Change-Id: I0d36f972fcb9888ed2be4ec9360bbcab13c25c5d

guix/import/cran.scm

@@ -85,6 +85,21 @@
 (define %input-style
   (make-parameter 'variable)) ; or 'specification
 
+(define (format-inputs inputs)
+  "Generate a sorted list of package inputs from a list of upstream inputs."
+  (map (lambda (input)
+         (case (%input-style)
+           ((specification)
+            `(specification->package ,(upstream-input-name input)))
+           (else
+            ((compose string->symbol
+                       upstream-input-downstream-name)
+             input))))
+       (sort inputs
+             (lambda (a b)
+               (string-ci<? (upstream-input-name a)
+                            (upstream-input-name b))))))
+
 (define (string->licenses license-string license-prefix)
   (let ((licenses
          (map string-trim-both
@@ -177,9 +192,7 @@ package definition."
     (()
      '())
     ((package-inputs ...)
-     `((,input-type (list ,@(map (compose string->symbol
-                                          upstream-input-downstream-name)
-                                 package-inputs)))))))
+     `((,input-type (list ,@(format-inputs package-inputs)))))))
 
 (define %cran-url "https://cran.r-project.org/web/packages/")
 (define %cran-canonical-url "https://cran.r-project.org/package=")