The 2.2 modules is planned to be made available via composer as a part of RC2 onwards. (In one sense, that is almost the definition of RC2.) The RC1.* releases have only been on GitHub. If you want to start planning now, you can look at the GitHub repo and docs - otherwise wait until August 14th for RC2 composer repo access. (Being in software for a while, I would add an unofficial comment of don't be shocked if something goes wrong and this slips a little, but things are looking on track from what I can see.)
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As background, we have been trying to get the code out as early as possible for those who want to start early, but there is nothing wrong starting from RC2 onwards. It will be more stable than RC1 (more bugs fixed) as we go through our own regression processes. But if you want maximal runway, all the code is available today (just not as convenient as when we hit RC2 with Composer downloads). Otherwise, wait for RC2 when the code will be available via Composer.
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And yes, we are encouraging people to start early because that will give us more time to address any issues reported back to us. We want to get the release out, but are doing what we can to make the code available to extension developers with as much notice as possible.