Zpython got renamed to libzpython

Not that I wanted this: cmake cannot be told I do not want `lib` prefix. `zsh/` 
prefix in any case should have been removed: zpython is not pretending it is 
a part of zsh now. Keeping old name for those who are still using zpython branch 
(i.e. nearly everybody since zpython got moved less then an hour ago).
Merge branch 'new-zpython-name' into develop