rackunit/docs-complete: move to the "racket-index" package

Modules are not supposed to move to different packages, anymore, so
I'm breaking a rule. The problem is that having `rackunit/docs-complete`
in "scribble-lib" made "scribble-lib" depend on "racket-index", and
that's an undesirable dependency. (Installing Scribble shouldn't
force you to have Racket documentation.) I should have noticed
the problem when I moved `setup/xref` to "racket-index", but I missed
it.

As far as I can tell, there's only one package on pkgs.racket-lang.org
that isn't in the main repository and uses `rackunit/docs-index`. The
package doesn't correctly declare its dependencies, anyway.

original commit: d33c2252e10dbeb22e49c85f03707345e2756ce5

pkgs/rackunit-pkgs/rackunit-doc/info.rkt

@@ -7,7 +7,8 @@
 (define pkg-desc "RackUnit documentation")
 
 (define pkg-authors '(noel ryanc))
-(define build-deps '("racket-doc"
+(define build-deps '("racket-index"
+                     "racket-doc"
                      "rackunit-gui"
                      "rackunit-lib"
                      "scribble-lib"))