Reorg probe code

[tomwilkie]

Ideas:

• directory per data source (docker, process, endpoints)
• tagger interface & topology generating interface.
• break up the docker tagger into different responsibilities.

[tomwilkie]

As part of #237 I moved all the docker code into probe/docker, and introduced a Reporter interface. I think we can make everything either a Tagger or a Reporter.

[tomwilkie]

Thought: we could introduce a "process" module, and disaggregate pidtree into a process walker, a pidtree implementation (using the walker), and a process reporter (using the walker)

[tomwilkie]

#270 reorganised the process module.

[tomwilkie]

I think all thats left is to:

• give the weave tagger a similar treatment (weave module, separate tagger and reporter)
• consider moving origin_host_tagger into host module