[Dlang-internal] Removing ddmd. prefix from dmd module names

[Walter Bright via Dlang-internal]

The dmd build has a problem - all the .d files have to be specified on the same 
command line. The trouble is the prefix ddmd. on all the imports, yet there is 
no ddmd directory. D is designed to have the path/file structure match the 
package/module structure.

   https://github.com/dlang/dmd/pull/6249

illustrates the change. Another option is to move the files into a subdirectory 
named 'ddmd'.

It's worth some wider discussion.