Break the v2 manifest across modules

[itowlson]

My super smart sports watch informs me that I have now spent the equivalent of three years of my life searching around the v2.rs manifest file trying to find the type / enum case / whatever the heck it is I want. This is in some measure down to greatly enlarged doc comments that we did for JSON schema, which are good and splendid but mean you can only have like 2 declarations on screen at a time.

I propose breaking the file up, and as a sighting shot I've done it by top-level section, with the exception of a couple of things that are used across sections. So you end up with something like this:

which to me feels like it will be easier to locate "oh that's where the component manifest goes and now there are only like three things in there." Although maybe having all the files collapsed under the v2 folder will drive me to even greater heights of navigation rage, who knows. And component.rs and dependency.rs are still 350 lines each which is a bit gross (curse you, biz logic on its associated type). Perhaps I should take the hammer to them yet further? Anyway open to feedback as ever.

There are no actual code changes: this is pure cut-paste-use-format. All existing pub exports from the v2 module remain pub exported from the v2 module; no new pub exports have been added (at least if they have it's a mistake). I left the tests in the base v2 file because they don't get in the way so much.