proposal: private module-level import for faster compilation

[Timothee Cour via Digitalmars-d]

this simple example shows this feature would provide a 16X 
speedup.

time dmd -c -o- -version=A -I$code main.d
0.16s

time dmd -c -o- -version=B -I$code main.d
0.01s


---main.d:
module tests.private_import.main;
import tests.private_import.fun;
void test(){}
---

---fun.d:
module tests.private_import.fun;
version(A) import std.datetime;
//version(C) private import std.datetime;
void foo(){
// same as version(C) if this feature were implemented
version(B) import std.datetime;
}
---