Add identifiable Python include directory name for Python wrappers

[schnellerhase]

Currently the python module installs the header files into the (include) path python/dolfinx/wrappers, which is then accessible through dolfinx.get_include.

This however drops the necessity/option of a name giving include path for dolfinx extension modules. For example the array.h file of the dolfinx wrappers is then included in a third party lib as <array.h> obscuring its origin. This can be for example seen in action in dolfinx_mpc and multiphenicsx .

This PR introduces an additional sub directory dolfinx_wrappers for the installed headers, so the new include for the pyhton wrapper headers adds the descriptive additional prefix <dolfinx_wrappers/...>.

Also, removes file python/dolfinx/wrappers/__init__.py, which is superseded by python/dolfinx/__init__.py::get_include, and moves py_to_cpp_coeffs into the dolfinx_wrappers namespace (previously in global scope).