My idea was to make Phobos2 a subpackage, but I suppose this works as well.
I'm not entirely sure about druntime, it looks like it's just include files
(the sources do produce a libdruntime-linux64.{a,so} files, but they're not
part of the official RPM), so it's probably worth making a dmd-devel package
for that.
Oh, and it's probably a good idea to turn off building for non-x86 targets, as
DMD doesn't seem to support those yet.