Fix several issues with handling of numpy in CMake.

Issues were:
* Abusing of `WITH_PYTHON_INSTALL_NUMPY` by both Audaspace and
  Mantaflow.
    - `PYTHON_INSTALL` options only decide whether we copy python (and
      some extra modules) in our Blender installation. On linux it
      makes much more sense to use global python installation.
    - Now we have instead a proper `WITH_PYTHON_NUMPY`
* Bad assumptions regarding path of headers relative to path of python
  module.
    - In current Debian testing, modules are under `python3.9`
      directory, while headers are under `python3` directory.
    - Now we properly `find_path` for headers as well, modifying
      `find_python_package` to take an optional argument for headers.

Note that the required changes done to `extern` libraries are in
blender-specific files that do not exist upstream.

Differential Revision: https://developer.blender.org/D9773

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