fix(ng-dev): publishing fails if pre-release is cut for new minor/major

[devversion]

Currently publishing fails if a pre-release is cut for an upcoming
major/minor. This is because the next release-train has some
special logic where its version is already bumped, but the
version has not been released. Our release notes logic currently
when building a pre-release for next, looks for a tag named
similar to the version in the package.json. This will always fail.

To fix this, we consult the release trains for building up
the Git range that is used for the release notes. We still
rely on tags, but we determine the tags from the active release
trains (which is more robust than relying on the package json value).

For the special next release-train situation, we build up the
release notes from the most recent patch to HEAD of next. While
doing this, we also dedupe any commits that have already landed in
patch, RC, or FF releases.

[josephperrott]

Overall LGTM

Probably worth having @clydin and/or @AndrewKushnir take a look as well.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[josephperrott]

LGTM

[devversion]

@jelbourn I've merged this to unblock other work on the release tool (and to make the tool usable as soon as possible). I'd still appreciate your review on the PR when you get time (especially on the tests for the changelog generation). No rush.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}