providers: Disable if g:loaded_* exists.

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

runtime/autoload/provider/python.vim

@@ -4,7 +4,7 @@
 " Associating the plugin with the Python host is the first step because plugins
 " will be passed as command-line arguments
 
-if get(g:, 'loaded_python_provider', 0)
+if exists('g:loaded_python_provider')
   finish
 endif
 let g:loaded_python_provider = 1