Reorganize the way we import CPython code

We're importing lots of code from a number of the CPython directories,
so it makes a lot more sense to mirror their directory layout rather than
the original scheme which was based on thinking we'd be importing just
.py files, and from multiple other implementations.  Once we started to
add other directories, we ended up with weith directory names like
"lib_python/2.7_Python" -- which is now a much more reasonable
"cpython2.7/Python".

Having the same directory layout also helps with getting the CPython
code to work, since in some ways it can sometimes encode the directory
layout (traverse to another directory in an #include, sysconfig.py).

Sorry for the 750kloc diff, but it had to happen some day...