Make patch discovery and parsing pluggable.

Everyone seems to have their own preference for how patches are managed. Out of the box, this module supports my own personal preference, but there's a lot of demand for slightly different ways of working in the issue queue, so I think it's work making the module's workings pluggable, so people can choose one of the existing ways of working, or implement their own.

The existing two mechanisms for discovering patches in patch_status should be ported to new pluggable mechanism. One of them will be the default on a new install. Only one pluggable patch discovery mechanism will be supported at a time. (I think if you need more than one, you should probably organise things better.)