gnu: python-waf: Use a proper directory structure.

Fixes <https://bugs.gnu.org/25634>.

* gnu/packages/python.scm (python-waf): Replace custom 'install phase to
install "waf" binary into the bin directory. Remove the 'wrap phase.
* gnu/packages/video.scm (mpv)[arguments]: Adjust the 'setup-waf phase
accordingly.