Message 87494 - Python tracker

with --inplace etc. build_ext.get_outputs returns wrong extension paths;
tries to computes them out of the air *again* and ..

Tools which need to further know the produced module file, e.g.
pyximport (Cython) do crash then ...

patch see attachment: stores and uses the real path in use.
writes a new attribute ext_filename to Extension object - should it be
underscored _ext_filename or ... ?