Port to Buildstream 2.0

This is a rebase of the bst 2 branch on top of master. This isn't yet ready but it's working and we can start discussing things and finalizing stuff.

I'd like to formally propose that we merge this now. I've squashed together some commits and reworded commit messages, it should be ready to review.

The only remaining question is should we merge this now, or should we wait until #1448 (closed) is fixed. I'd say let's merge now, as it would unblock further work on bst 2 (this MR is already huge, at +534 -1335 in 97 files). It should help us also discover other bugs we might hit.

Since this is so huge, let's aim for three approvals.

Closes #1383 (closed)