Use backslash in plugin name when loading.
Remove plugin namespace aliasing and shortening.

Aliasing a plugin namespace in app won't work as the plugin itself
could be using plugin notation with hardcoded name.
Remove usage of purged method Plugin::namespace().
Fix translation file loading for vendor namespaced plugins.
Fix camelizing of vendor namespaced plugin names.
Fix typo in comment.
Merge pull request #4236 from ADmad/3.0-plugin

3.0 plugin