There's a tiny, but extremely useful enhancement request, that resides in bugzilla for a long time now. It proposes the possibility to have modules named "package", in which case they're imported using their package name alone:
mylib
core.d
aux.d
package.d
// main.d:
import mylib.core;
import mylib.aux;
import mylib;
In this case adding UDAs to packages essentially means adding UDAs to the "package" module.
And adding UDAs to any modules would look like this:
[MyUda] module mylib.core;
Since the module-level import statements affect the entire module, the later imports would be enough to make the MyUda type visible at the top.