I think we should make the upgrade shell a separate plugin.

After upgrading it's code that isn't useful; it's not currently tested (or infact: working at all for upgrading plugins) and giving it appropriate test coverage means adding appropriate test stubs, which it a lot of noise.

I'm most of the way through [restructuring the existing upgrade shell](https://github.com/AD7six/cakephp/compare/cakephp:3.0...3.0-upgrade) - the more I go on though, the more it seems like bloat for cakephp/cakephp.

Opinions?