DUB: Sometimes generates .di file and sometimes do not.

[BoQsc]

To test the behaviour of static library.

**program.d**
```
module program;
import std.stdio;
import library;

void main(string[] args)
{
     writeln("func(3) = ", library.func(3));
}
```

**Command Line:**

```
dmd "./program.d" "./builds/library.di" "./builds/library.lib" 
-ofProgram.exe
Program.exe
```


**Output:**
```
func(3) = 4
```