FindPython: new implementation

This MR address a large part of current known problems, as described in #16142 (closed) regarding Python handling. The implementation follows the approach proposed by @ben.boeckel except that the module FindPython search for versions 3 and, if version 3 is not found, for version 2.

I also integrate the support of IronPython on Windows. Other python variants (PyPy, etc...) are not supported for now.

I try to address a large part of scenarii and I test these new modules on various environments:

• MacOS: system python + homebrew (standard python + ActivePython + Anaconda)
• Linux: system python + custom installations
• Windows:
  • WIN32 : Official Python + ActivePython + Anaconda + IronPython
  • MSYS
  • CYGWIN