Don't try and make a module that doesn't include main

[benl]

Added by Jared:

The easiest way to reproduce this is to compile a file other than Main.ds with the -m flag. After ld fails to find _main, ddc panics.

ddc: PANIC in Main.Link
        invokeLinker: link failed
             objects:

It then lists all the object files in the standard library and advisees to report the panic.