runtime: Refer to plugins running outside Nvim as "remote plugins"

- Rename autoload/rpc to autoload/remote
- External plugins are now remote plugins
- External plugins directory is "rplugin"

runtime/plugin/rplugin.vim

@@ -0,0 +1,5 @@
+if exists('loaded_remote_plugins') || &cp
+  finish
+endif
+let loaded_remote_plugins = 1
+call remote#host#LoadRemotePlugins()