Order dependent module loading with `Distributed`

Discovered by @ararslan in his quest to get Nanosoldier back online.

In distributed mode using X should make X available on the worker nodes as a root module so that remotecall(X.f) works without a @everywhere X.

This correctly works:

julia> using Distributed

julia> addprocs(2)
2-element Array{Int64,1}:
 2
 3

julia> @everywhere begin
       function log_require(mod::Base.PkgId)
         @info "Loading pkg $mod $(Base.root_module_exists(mod)) on proc $(myid())"
       end
       push!(Base.package_callbacks, log_require)
       end

julia> using BenchmarkTools
[ Info: Loading pkg JSON [682c06a0-de6a-54ab-a142-c8b1cf79cde6] true on proc 1
[ Info: Loading pkg JSON [682c06a0-de6a-54ab-a142-c8b1cf79cde6] true on proc 2
[ Info: Loading pkg JSON [682c06a0-de6a-54ab-a142-c8b1cf79cde6] true on proc 3
[ Info: Loading pkg BenchmarkTools [6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf] true on proc 2
[ Info: Loading pkg BenchmarkTools [6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf] true on proc 3
[ Info: Loading pkg BenchmarkTools [6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf] true on proc 1

If the user says using X before addprocs we no longer trigger the callback on a subsequent using X,
thereby triggering https://github.com/JuliaLang/julia/pull/28857#issuecomment-415590074

julia> using Distributed

julia> using BenchmarkTools

julia> addprocs(2)
2-element Array{Int64,1}:
 2
 3

julia> @everywhere begin
       function log_require(mod::Base.PkgId)
          @info "Loading pkg $mod $(Base.root_module_exists(mod)) on proc $(myid())"
      end
      push!(Base.package_callbacks, log_require)
      end

julia> using BenchmarkTools

julia> @everywhere using BenchmarkTools
[ Info: Loading pkg JSON [682c06a0-de6a-54ab-a142-c8b1cf79cde6] true on proc 2
[ Info: Loading pkg JSON [682c06a0-de6a-54ab-a142-c8b1cf79cde6] true on proc 3
[ Info: Loading pkg BenchmarkTools [6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf] true on proc 2
[ Info: Loading pkg BenchmarkTools [6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf] true on proc 3