Update to support next version of BuildStream

Benjamin Schubertrequested to merge
bschubert/update-for-source-consistency into master

This updates the plugins to support the not-yet-released version of BuildStream that contains breaking changes for sources.

The tests here are run against the previous version of BuildStream.

However, we can see https://gitlab.com/BuildStream/buildstream/-/jobs/404431555 on BuildStream itself work correctly.

Edited by Benjamin Schubert

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