Print a warning when mixing $EBPYTHONPREFIXES and $PYTHONPATH modules

[Flamefire]

$PYTHONPATH takes precedence which might lead to unexpected or wrong packages being used especially when a dependency includes a newer package than another dependency.
As it is very hard to find this as the cause print a warning when this situation is found.

I ran into this while building scipy of the SciPy-bundle which includes patches that require a newer Cython as the build dependency then what is available in the Python-bundle-PyPI module. As only Python-bundle used $PYTHONPATH the package from the Cython module was ignored even though which cython points to the right binary.

The error wasn't helpful:

Error compiling Cython file:
------------------------------------------------------------
...
    cdef __Pyx_memviewslice memslice
    cdef Py_ssize_t itemsize
    cdef bint dtype_signed
    cdef char kind
    itemsize = -1
    cdef bint ___pyx_const int8_is_signed
                          ^
------------------------------------------------------------

(tree fragment):16:27: Syntax error in C variable declaration

The least we can do is to detect such issues and warn the user.

The check is triggered for direct dependencies when both $PYTHONPATH and $EBPYTHONPREFIXES are set and the Python module is loaded.
I only check for modules that have a lib/python*/site-packages folder and specifically exclude the Python module which sets $PYTHONPATH for the site-customize.py
This should catch most false positives while still catching enough real issues