use qualified name when aliasing types

[skallweitNV]

Use qualified name when writing type aliases.

If you have some bindings like this:

nb::class_<A> a(m, "A");
nb::enum_<E> e(a, "E");
nb::class_<B> b(m, "B");
b.attr("E") = a.attr("E");

you currently get:

class A:
    class E(_Enum):
        ...

class B:
    E: TypeAlias = E  

with this fix you get:

class B:
    E: TypeAlias = A.E

[wjakob]

LGTM, thanks!