This changes the publishing of archives so that it happens on the chrome_infra bots when they build a packaged branch instead of as part of the dev_roll process.
It uses the tagged version in the branch, and leaves the git repo that it clones checked out on the branch and hash used to build the package.
It updates metadata located at gs://flutter_infra/releases/releases_.json (where is one of macos, linux, or windows) once published, since it would be complex to do the proper locking to keep them all in one shared .json file safely.
A separate [change to the chrome_infra bots](https://chromium-review.googlesource.com/c/chromium/tools/build/+/902823) was made to instruct them to build packaged for the dev, beta, and release branches (but not master anymore).
This script will update release metadata in the cloud, and copy the already-built package to the right location and name on cloud storage.
The release metadata will be located in gs://flutter_infra/releases/releases.json, and the published packages will end up in gs://flutter_infra/releases/<channel>/<platform>/flutter_<platform>_<version><archive suffix>, where <channel>, <platform>, <version>, and <archive suffix> are determined by the script.
At the moment, it only supports dev rolls, but (once we know how those will work) should easily support beta rolls as well.
Generate the "version" file from git tags.
Remove the old VERSION file and mentions of versions in pubspec.yaml files.
Replace the old update_versions.dart script with a new roll_dev.dart script.
Update "flutter channel".
Update "flutter upgrade", including making it transition from alpha to dev.
Update "flutter --version" and "flutter doctor".
