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All targets provided by pybind11:

* pybind11::module - the existing target for creating extension modules
* pybind11::embed - new target for embedding the interpreter
* pybind11::pybind11 - common "base" target (headers only)

Makes room for an eventual pybind11::embedded target.

See the documentation for a description of the options.

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