Generally, when starting a new branch packages are not copied over
to avoid wasting disk space. So, old packages will be still pulled
in from old branch directories (under packages/). This commit adds
a function that automatically flush back old-branch packages into
the current one.
Let's say we have just bumped our repo from branch 1 to branch 2.
We have 3 packages into it that are still placed inside /packages/1.
Everything works until we decide to drop support from branch 1, in
this case, we would have to rebuild all these packages wasting a lot
of time. With "flushback" this is done automatically without touching
packages integrity.
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