Use Vim's pythonx mechanism

Move jedi_vim.py and the jedi submodule into pythonx, which gets added
to Vim's internal sys.path.  While jedi cannot be imported directly from
there, it still makes sense for consistency.

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "jedi"]
-	path = jedi
+	path = pythonx/jedi
 	url = https://github.com/davidhalter/jedi.git