Add a BUILD_INTERFACE and a pybind11::pybind11 alias for the interface
library to match the installed target.
Add new cmake tests for add_subdirectory and consolidates the
.cpp and .py files needed for the cmake build tests:
Before:
tests
|-- test_installed_module
| |-- CMakeLists.txt
| |-- main.cpp
| \-- test.py
\-- test_installed_target
|-- CMakeLists.txt
|-- main.cpp
\-- test.py
After:
tests
\-- test_cmake_build
|-- installed_module/CMakeLists.txt
|-- installed_target/CMakeLists.txt
|-- subdirectory_module/CMakeLists.txt
|-- subdirectory_target/CMakeLists.txt
|-- main.cpp
\-- test.py
