Add a wee bit of infrastructure to improve how we deprecate broken

packages from branch to branch:

When a package is considered broken (for whatever reason) at branch
time, we'll mark it with BROKEN_IN=name-of-branch. At the next
branch, we can easily determine which ones have remained broken and
remove them.

BROKEN_IN is visible to users iff a build fails, when it appends a
warning message to the above effect.

With feedback from dillo. Reviewed by jlam.

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