Extract code from batches package into smaller sub-packages

Created by: mrnugget

This is the first step towards cleaning up this code base a bit.

It extracts code into smaller sub-packages:

• executor
• mock
• git
• log
• service
• workspace

And it renames some of the types so they make more sense in this new structure: batches.ExecutorOpts to executor.Opts for example.

This doesn't magically fix problems, but it makes it easier to see where dependencies and strong coupling exists.

And it's only the first step: there's still a few TODOs that I added. Next I want to clear up the interplay between service and executor and the main package.

I opened this PR to make reviewing easier: it doesn't change behaviour but only renames and moves things