The exists('g:loaded_foo') convention is rather common, and may be
relied upon in some cases. It's also very unlikely that a user or plugin
has any reason to set g:loaded_foo to zero, so the principle of least
surprise can be brushed aside here.
https://github.com/neovim/neovim/issues/6107#issuecomment-279532143
`provider#pythonX#Error` and `provider#pythonX#Prog` are currently meant
for debugging only (the error message is not being used), and should
therefore be defined always, especially in case of errors.
Ref: https://github.com/neovim/neovim/pull/2549#issuecomment-98607580
Clipboard is implemented with platform-specific shell commands, and python is
implemented with the external plugin facility (rpc#* functions). The
script_host.py file(legacy python-vim emulation plugin) was moved/adapted from
the python client repository.
