refacto: abstract TreeExplorer in core.lua

lua/nvim-tree/view.lua

@@ -1,5 +1,7 @@
 local a = vim.api
 
+local core = require "nvim-tree.core"
+
 local M = {}
 
 M.View = {
@@ -343,7 +345,7 @@ function M._prevent_buffer_override()
 end
 
 function M.is_root_folder_visible()
-  return TreeExplorer.cwd ~= "/" and not M.View.hide_root_folder
+  return core.get_cwd() ~= "/" and not M.View.hide_root_folder
 end
 
 local DEFAULT_CONFIG = {