profiles: Add 'packages->manifest' procedure.

* guix/profiles.scm (packages->manifest): New procedure.

guix/profiles.scm

@@ -80,6 +80,7 @@
 
             profile-manifest
             package->manifest-entry
+            packages->manifest
             %default-profile-hooks
             profile-derivation
             generation-number
@@ -172,6 +173,18 @@ omitted or #f, use the first output of PACKAGE."
      (dependencies (delete-duplicates deps))
      (search-paths (package-native-search-paths package)))))
 
+(define (packages->manifest packages)
+  "Return a list of manifest entries, one for each item listed in PACKAGES.
+Elements of PACKAGES can be either package objects or package/string tuples
+denoting a specific output of a package."
+  (manifest
+   (map (match-lambda
+         ((package output)
+          (package->manifest-entry package output))
+         (package
+           (package->manifest-entry package)))
+        packages)))
+
 (define (manifest->gexp manifest)
   "Return a representation of MANIFEST as a gexp."
   (define (entry->gexp entry)