make pkg-links: fix to allo "racket" as a dependency

Probably only the "base" package should rely on a "racket" version,
while other packages should rely on a "base" version.
move `setup/xref` from "scribble-lib" to "racket-index"

The `setup/xref` library conceptually belongs in "racket-index",
and moving the library avoids a cross-package use of a private
module.

Merge to v6.0