DIP11: Automatic downloading of libraries

[Adam D. Ruppe]

bearophile:
> Isn't this also an argument for the (ancient request of) inclusion
> of a normal build feature into the D compiler?

I think if the compiler is going to be downloading files, it is
necessarily looking for those files... thus it'd become a bit of
a smarter build tool as a side effect of this change.

It'd be silly if it said "this module is missing, and I'll download
it, but I refuse to actually look at it once it's downloaded!"