Module is in multiple defined

[Max Samukha]

Following the discussion of module declarations, there is still an
issue that dates back to 2005 and obviously has been ignored since
then. Consider this:

file 1: pack/module1.d
------------------------
import pack.module2;


file 2: pack/module2.d
------------------------
import pack.module1;

Compiling the above with 'dmd pack/module1 pack/module2' gives weird
messages like 'module module1 is in multiple defined'.

Adding explicit module declarations fixes the issue. It was discussed
here:
http://www.digitalmars.com/d/archives/digitalmars/D/learn/1302.html

This issue effectivly renders unnamed modules unusable.