Re-check dependencies for META6.json and reevaluate its role

[JJ]

Problem or new feature

As part of the route to 2.0 #2983 we have spun off several utilities; eventually p6doc will have to be moved since @softmoth has already created Rakudoc. The META6.json should contain only what's needed for... not clear what, really. Most dependencies are used for testing, but that's mainly for developers.

Suggestions

As long as rakudoc has been spun off, we should rethink the whole META6.json thing. Is this a repo for doccers? Is it for the end user? Should we eventually create a docs-only repo that would be re-used for doccers?

As an immediate step, simply check the dependencies that are no longer used in the repo at all.

[coke]

p6doc has been removed, and by the time it was, it wasn't listed in the META6.json

[coke]

Two bits of cleanup I can see: move where the dependencies are declared (build/test?/run - if some are xt only, no need to force a user to install them, sure), and we can decide if we want to expose our internally used modules - but as we move things out of the repo, that will naturally get sorted out.

[coke]

The dependency cleanup has already been done at this point, items are in test-depends or build-depends. there are no runtime deps.

Even if we're not installing the documents from the repo (an open question), we still need META6.json for editors and the build.