Split annotations and processor in different projects

[cristcost]

Split annotations and processor in different projects, in order to allow managing the propagation of dependencies of the processor (needed only by projects generating code) and the annotations independently

[tbroyer]

You must shade your dependencies in the processor if you don't want to:

• mess up with the projects' own dependencies (or risk a conflict), when the processor dependency participates in conflict mediation/resolution (e.g. when using Maven, given that you cannot put the processor in its own independent "scope")
• risk conflicts with other annotation processors
• risk users depending on the processor's dependencies and failing at runtime when they're absent

That said, having distinct artifacts is a good idea too (see discussion in google/auto#268)