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The behavior is definitely due to the calling sequence of
add_submodule and load_module in import_submodule. (like
following)

...
m = load_module(fullname, fp, buf, fdp->type, loader);
Py_XDECREF(loader);
if (fp)
    fclose(fp);
if (!add_submodule(mod, m, fullname, subname, modules)) {
    Py_XDECREF(m);
    m = NULL;
}
...

For "importing package.x;do_something(package.x)" from
within package.x to be possible,  add_submodule should be
done before load_module, since in load_module,  not only the
module is load, but also executed by PyImport_ExecCodeModuleEx.
So, if we make a module and call add_submodule before
load_module, import ing package.x and using it is possible.
(like following)

m = PyImport_AddModule(fullname);
if (!m) {
        return NULL;
}

if (!add_submodule(mod, m, fullname, subname, modules)) {
        Py_XDECREF(m);
        return NULL;
}

m = load_module(mod, fullname, fp, buf, fdp->type, loader);
Py_XDECREF(loader);

but above would make test_importhook fail because in
IMP_HOOK case, module object is created by
PyObject_CallMethod(... "load_module"..), not by calling
PyImport_AddModule. So, we cannot know about the module
before that  method calling is returned. Thus, in IMP_HOOK
case, load_module would not use the already-created module
by PyImport_AddModule, but would make a new one, which is
not added as submodule to its parent.

Anyway, adding another add_submodule after load_module would
make import-hook test code to be passed, but it's a lame
patch since in IMP_HOOK case, import package.x in
package/x.py cannot be done.

So, for the behavior to be possible, I think load_module
should be explicitly separated into two function -
load_module, execute_module. And then we'll load_module,
add_submodule itself to its parent and then execute_module.
There does not seem to be any hack that touches only limited
places, so I think this bug(?) will stay open for quite long
time. =)