make extension not load twice on workers (JuliaLang/julia#49441)

Author: KristofferC

src/Distributed.jl

@@ -76,6 +76,9 @@ function _require_callback(mod::Base.PkgId)
         # broadcast top-level (e.g. from Main) import/using from node 1 (only)
         @sync for p in procs()
             p == 1 && continue
+            # Extensions are already loaded on workers by their triggers being loaded
+            # so no need to fire the callback upon extension being loaded on master.
+            Base.loading_extension && continue
             @async_unwrap remotecall_wait(p) do
                 Base.require(mod)
                 nothing