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

[kdevel]

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?