Traditional Rails plugins should be deprecated in 3.0

Continuing from my previous ticket #3126 script/generate plugin should be deprecated/removed for 3.0 the entire plugin infrastructure should be deprecated in Rails 3.0 and removed in Rails 3.1, replacing it with the gem loading mechanism for incorporating reusable extensions.

I'm sure it's not as easy as all that -- I reckon there's currently a few things that plugins do which aren't supported with the gem loading infrastructure -- but there's a huge win from getting everything to use a package management system which supports versioning and dependencies.

Deprecation messages in 3.0 will encourage plugin authors to replace their existing plugins with gems, which is definitely for the win.