PC requires all release channels to have tags available

[FichteFoll]

Via wbond/package_control_channel#7834.

Currently, PC wants a release for each channels and fails early otherwise. We should probably check that every package has a tag here as well.

Note that the reviewer integration in https://github.com/wbond/packagecontrol.io has custom download code and tag verification iirc and only uses this package for the actual package.

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I might switch package_control_channel to use Github Actions and do everything on its own, so the test runner could be run with that instead on the PC server.