Remove "circular require considered harmful" error

rails/generators should not load rails/generators/base since the
latter is loading it.
Set the destination_root for plugins

Since the plugin generator is relying on the application one, we need to
define the proper root when creating the plugin. The AppBase class is
not setting the destination_root (this is done via the AppGenerator)
Fix some failing tests for Rails::Paths

Since we are not using the File.exists? alias which raises a warning on
current ruby trunk, few stubs are wrong.
Merge pull request #12733 from robin850/fix-railties-build

Fix railties build