Replacing a Julia module is not handled well

[fingolfin]

I just run into this while testing some code:

gap> JuliaEvalString("module foo  x = 1 end");
<Julia: Main.foo>
gap> Julia.foo.x;
1
gap> JuliaEvalString("module foo  x = 2 end");
WARNING: replacing module foo.
<Julia: Main.foo>
gap> Julia.foo.x;
1

So what happens is that Julia.foo is cached, and so is not updated when the underlying module changes. We should deal with that better.