DIP11: Automatic downloading of libraries

[Adam D. Ruppe]

> One other interesting aspect is that the string literal can be
> CTFE-constructed,

Oh, or it could be in version {} blocks. I like that.

I think we should actually whip up a working model. It needn't be
a compiler feature at this point - we can use
pragma(msg, "BUILD: " ~ param) for now and have a helper program
scan dmd's output.