connected pybind11 modules

I think my problem is not necessarily a bug, but asking before on SO, gitter and an old related issue, didn't lead to solutions which solved the issue. Futhermore i think this use case might come up more frequently and can thus be of use for others if documented.

Issue description

Am trying to create and bind the following two modules with pybind11:

• point: can be called directly
• line: can be called directly and also uses point module.
  Trying to use use the line module leads to an undefined symbol error:
  Symbol not found: __ZN5pointC1Eddd

Reproducible example code

The original setup is as follows:

point.h:

#ifndef UNTITLED1_POINT_H
#define UNTITLED1_POINT_H

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>

namespace py = pybind11;

class Point {

private:
    double m_x;
    double m_y;
    double m_z;

public:
    Point()= default;
    Point(double x, double y, double z);
};

PYBIND11_MODULE(point, m) {

    py::class_<Point>(m, "Point")
            .def(py::init<double, double, double>());

}

#endif //UNTITLED1_POINT_H

point.cpp:

#include "point.h"
Point::Point (double x, double y, double z){
    m_x = x;
    m_y = y;
    m_z = z;
}

line.h:

#ifndef UNTITLED1_LINE_H
#define UNTITLED1_LINE_H

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>

namespace py = pybind11;

#include "point.h"

class Line {
private:
    Point m_p1;
    Point m_p2;
public:
    Line(Point p1, Point p2);
};

PYBIND11_MODULE(line, m) {

    py::class_<Line>(m, "line")
            .def(py::init<Point, Point>());

}

#endif //UNTITLED1_LINE_H

line.cpp:

#include "line.h"

Line::Line(Point p1, Point p2) {
    m_p1 = p1;
    m_p2 = p2;
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.0)
project(untitled1)

set(CMAKE_CXX_STANDARD 11)

add_subdirectory(pybind11)

pybind11_add_module(point point.cpp)
pybind11_add_module(line line.cpp)

Now the following python code is run:

from point import point
from line import line

p1 = point(1, 2, 3)
p2 = point(3, 4, 5)

l = line(p1, p2)

leading to a undefined symbol error:
Symbol not found: __ZN5pointC1Eddd

I have also tried the following lines in the cmake file:
pybind11_add_module(point SHARED point.cpp)
pybind11_add_module(line line.cpp)
target_link_libraries(line PRIVATE point)

Also tried adding different linking operations:
py::module::import("point");
py::object point = (py::object) py::module::import("point");
py::module point = py::module::import("point");

Start with issue #1391 and reproduce the two-module example using point.h, point.cpp, line.h, line.cpp, and CMakeLists.txt. Review the existing pybind11 documentation for module builds and linking, then document the supported setup and verify it with the provided Python import sequence and undefined-symbol case.