package modules and how to generate a shared library plus .di file (I)

[kdevel]

Given the functions void foo() and void bar() in their source 
files mymod/foo.d and mymod/bar.d. Also I have a

mymod/package.d
```
module mymod;

public import foo : foo;
public import bar : bar;
```

and a client to the library:

main.d
```
import mymod;

void main ()
{
    foo;
    bar;
}
```

It compiles fine with

    $ dmd main.d mymod/foo.d mymod/bar.d

Now I want to link against libmymod.a. First I prepare the 
library:

    $ dmd -lib -oflibmymod.a mymod/foo.d mymod/bar.d

and then

    $ dmd main.d -L-L. -L-lmymod                                 
(*)
    mymod/package.d(3): Error: module foo is in file 'foo.d' which 
cannot be read
    import path[0] = .../linux/bin64/../../src/phobos
    import path[1] = .../linux/bin64/../../src/druntime/import

In order to make that Error go away I have to change

package.d
```
module mymod;

public import mymod.foo : foo;
public import mymod.bar : bar;
```

Then (*) works as expected. But why do I have to use the prefix 
"mymod.:" in the
library case?