provider: skip non-provider has() feature-names

We don't want to retry autoload sourcing (slow) for every random has()
query that finds it way to eval_call_provider().

test/functional/fixtures/autoload/provider/python.vim

@@ -0,0 +1,6 @@
+" Dummy test provider, missing this required variable:
+"   let g:loaded_brokenenabled_provider = 0
+
+function! provider#python#Call(method, args)
+  return 42
+endfunction