Added 'install_path' option for giving non-packagized module

Added 'install_path' option for giving non-packagized module
  distributions their own directory (and .pth file).
Overhauled how we determine installation directories in
  'set_final_options()' to separate platform-dependence and take
  'install_path' option into account.
Added 'create_path_file()' to create path config file when 'install_path'
  given.
Only run 'install_py' and 'install_ext' when, respectively, there are
  some pure Python modules and some extension modules in the distribution.