Remove reliance on App::objects() from Plugin::loadAll()

App::objects() is going away as it has a number of annoying flaws that
are not easily fixable. It cannot:

* Find prefix controllers.
* Find vendor namespaced plugins.
* Find entity classes etc.

In light of all this, it would be better if each of the callers
implemented the directory scanning logic they needed and App::objects()
was not a source of lies.

Refs #4256
Don't use App::objects() in command shell tools.
Don't use pass by ref variables.

I don't much care for pass by ref when there are simpler ways to write
the required code.
Remove App::objects() call.

Using bake templates now requires you to load a plugin. I don't see this
as a huge obstacle if you want to use templates from a plugin. This also
fixes issues where plugins not using `src/` would not work with
TemplateTask.
Remove App::objects().

Since it has no callers and frequently does the wrong thing, it no
longer needs to be around.

Refs #4256
Merge pull request #4279 from cakephp/no-app-objects

Remove App::objects()