Import module

[Adam D. Ruppe via Digitalmars-d-learn]

The best thing to do is to pass all the modules in the project to 
the compiler at once, so like:

dmd map/map.d steering/game_object.d

The import path could be changed to not include the steering/ 
folder - give it the folder that contains steering - but that 
still won't actually link without an extra step (you'd still have 
to compile the other module). Passing it all at once just works.

Alternatively, you might be able to run

rdmd map/map.d

if the folders are predictable enough too.