Fix shared action module in two layers (#51510)
Currently an "action module" (files with `"use server"` on top) can be
imported by both the server and client layers. In that case, we can't
fork that action module into two modules (one on the server layer, one
on the action layer), but only create it once on the server layer.
This ensures that the action module instance doesn't get forked.
Closes #50801.
fix NEXT-1265
