dmd -unittest -main -run: undefined reference to `_D1c12__ModuleInfoZ'

[kdevel via Digitalmars-d-learn]

In the current working directory b/ I have

package.d
```
module b;
import c;
void bar (string s)
{
   mkdir (s);
}
```

c/package.d
```
module c;
package import std.file;
```

   $ dmd -unittest -main package.d c/package.d

produces the binary "package" as expected, but

   $ dmd -unittest -main -run package.d c/package.d

fails with

   package.o:(.data.rel.ro+0x10): undefined reference to 
`_D1c12__ModuleInfoZ'
   collect2: ld returned 1 exit status
   Error: linker exited with status 1

Why this?