chore: make public api consistent

Summary:

Right now we have two ways to access the public api we have `require('ivy')`
and `vim.ivy. Each way has a different api that will cause some confusion.

Now both apis are the same so anyone that wants to integrate with ivy can do so
without having to figure out what one they need to use.

Test Plan:

The unit tests cover most of the work, I have also been using this locally for
quite some time now with now issues.

lua/ivy/controller.lua

@@ -1,5 +1,6 @@
 local window = require "ivy.window"
 local prompt = require "ivy.prompt"
+local utils = require "ivy.utils"
 
 local controller = {}
 
@@ -54,7 +55,7 @@ end
 
 controller.checkpoint = function()
   vim.api.nvim_set_current_win(window.origin)
-  controller.callback(window.get_current_selection(), controller.action.CHECKPOINT)
+  controller.callback(window.get_current_selection(), utils.actions.CHECKPOINT)
   vim.api.nvim_set_current_win(window.window)
 end