Relative imports

When including the the resulting Julia files from a.proto and b.proto inside a Julia module Julia complains that it's unable to locate Module A.  Using a relative import when building the Julia files fixes this issue however may not be the desired behaviour in all cases.

example (without relative imports):

```Julia
julia> module X
         include("a_pb.jl")
         include("b_pb.jl")
       end
ERROR: A not found
 in require at loading.jl:39
 in include at boot.jl:238
at b_pb.jl:3
```