.\main.d
.\foo.d

main.d:

module main;
import foo;
void main() { test(); }

foo.d:

module bar;
void test() { }

This is OK:

$ dmd -c main.d foo.d
> main.d(2): Error: module bar from file foo.d must be imported as module 'bar'

This should also fail to compile but it doesn't
$ dmd -c main.d
>