runtime: Reimplement python/clipboard providers in vimscript

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.

runtime/autoload/provider/python.vim

@@ -0,0 +1,28 @@
+" The python provider uses a python host to emulate an environment for running
+" python-vim plugins(:h python-vim). See :h nvim-providers for more
+" information.
+"
+" Associating the plugin with the python host is the first step because plugins
+" will be passed as command-line arguments
+if exists('s:loaded_python_provider') || &cp
+  finish
+endif
+let s:loaded_python_provider = 1
+let s:plugin_path = expand('<sfile>:p:h').'/script_host.py'
+" The python provider plugin will run in a separate instance of the python
+" host.
+call rpc#host#RegisterClone('legacy-python-provider', 'python')
+call rpc#host#RegisterPlugin('legacy-python-provider', s:plugin_path, [])
+" Ensure that we can load the python host before bootstrapping
+try
+  let s:host = rpc#host#Require('legacy-python-provider')
+catch
+  echomsg v:exception
+  finish
+endtry
+
+let s:rpcrequest = function('rpcrequest')
+
+function! provider#python#Call(method, args)
+  return call(s:rpcrequest, insert(insert(a:args, 'python_'.a:method), s:host))
+endfunction