Re-exported modules in index are not linked

[harpocrates]

Initially brought up in haskell/cabal#6066. Basically: modules that are re-exported currently aren't linked in the Hackage listing of modules in a package. Example: https://hackage.haskell.org/package/ghcjs-dom-0.9.4.0.

[adamgundry]

See also #577.

A workaround, discovered by @phadej and implemented for optics (well-typed/optics#465), is to manually upload a doc tarball that includes copies of the relevant files, because then Hackage will link them.