Let the D compiler print the dependencies.

[Frank Benoit (keinfarbton)]

> It sounds like a good idea but I think that printing depend: in front of the
> dependencies could complicate the use of the command. for example if you
> want to pipe it to another command. 

My idea behind that was, that printing the dependencies is independent
from doing compilation or other tasks (ddoc). Prefixing the message,
makes this possible.

The build tool can cache the dependencies from a previous run. First it
calls the compiler with all files from the last run. If it turns out,
that dependencies changed since the last run, it can rerun the compiler
with the new complete set of files. With this, it is possible to compile
most of the time without loading all source two times.
All other messages can be filtered and printed to stdout/stderr.