core: Continue PieCrust3 refactor, simplify pages. The asset pipeline is still the only function pipeline at this point. * No more `QualifiedPage`, and several other pieces of code deleted. * Data providers are simpler and more focused. For instance, the page iterator doesn't try to support other types of items. * Route parameters are proper known source metadata to remove the confusion between the two. * Make the baker and pipeline more correctly manage records and record histories. * Add support for record collapsing and deleting stale outputs in the asset pipeline.

---
config:
    publish:
        test_shell:
            type: shell
            command: this_command_doesnt_exist
args: publish --preview test_shell
---
config:
    publish:
        test_rsync:
            type: rsync
            destination: some_place_that_doesnt_exist
args: publish --preview test_rsync
---
config:
    publish:
        test_sftp:
            type: sftp
            host: some_host_that_doesnt_exist
            path: ~/some/path
            username: fake
            password: fake
args: publish --preview test_sftp