union: Ensure that the output is always a directory.

Fixes the creation of single-package profiles, reported by Ludovic Courtès.

* guix/build/union.scm (union-build): Add new internal procedure
  'union-of-directories' that always creates a directory, containing the code
  previously used only to merge multiple directories.  Call it from the
  multiple-directory case in 'union' and from the top-level 'union-build'.

guix/build/union.scm

@@ -108,8 +108,19 @@ the INPUTS."
        (call-with-values (lambda () (partition file-is-directory? inputs))
          (match-lambda*
            ((dirs ())
-            ;; All inputs are directories.  Create a new directory
+            ;; All inputs are directories.
-            ;; where we will merge the input directories.
+            (union-of-directories output dirs))
+
+           ((() (file (? (cut file=? <> file)) ...))
+            ;; There are no directories, and all files have the same contents,
+            ;; so there's no conflict.
+            (symlink* file output))
+
+           ((dirs files)
+            (resolve-collisions output dirs files)))))))
+
+  (define (union-of-directories output dirs)
+    ;; Create a new directory where we will merge the input directories.
     (mkdir output)
 
     ;; Build a hash table mapping each file to a list of input
@@ -133,19 +144,11 @@ the INPUTS."
                                    (reverse dirs-with-file))))
                      table)))
 
-           ((() (file (? (cut file=? <> file)) ...))
-            ;; There are no directories, and all files have the same contents,
-            ;; so there's no conflict.
-            (symlink* file output))
-
-           ((dirs files)
-            (resolve-collisions output dirs files)))))))
-
   (setvbuf (current-output-port) _IOLBF)
   (setvbuf (current-error-port) _IOLBF)
   (when (file-port? log-port)
     (setvbuf log-port _IOLBF))
 
-  (union output (delete-duplicates inputs)))
+  (union-of-directories output (delete-duplicates inputs)))
 
 ;;; union.scm ends here