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/typed-racket-pkgs/typed-racket-test/info.rkt

@@ -3,7 +3,8 @@
 (define collection 'multi)
 (define deps '("typed-racket" "typed-racket-more" "typed-racket-compatibility"
                "unstable" "unstable-2d"))
-(define build-deps '("scheme-lib"
+(define build-deps '("racket-index"
+                     "scheme-lib"
                      "base"
                      "racket-benchmarks"
                      "compatibility-lib"
@@ -14,8 +15,7 @@
                      "sandbox-lib"
                      "pconvert-lib"
                      "unstable-flonum-lib"
-                     "unstable"
-                     "scribble-lib"))
+                     "unstable"))
 
 (define pkg-desc "tests for \"typed-racket\"")