depends_on :python => 'module' allow user packages

Some prefer to install with `--user` into their
home. For example to avoid `sudo` with system's
python. Now Homebrew can use those modules to
satisfy a python dependency on a certain module.
doctor: share/python no longer needed in PATH

Check for HOMEBREW_PREFIX/share/python and python3
in PATH and tell about the new location and that
files from there can be removed. Also instructions
on how to upgrade python packages are given.