log.
Now that we can SKIPDIR, doing a first LISTING for most important dependencies
is not a trade-off for the full listing.
It allows dpb to start building most important stuff right away, so that
your machines start earlier.
Caveat: if that list of dependencies changes (ports renames), then you will
see errors because ports no longer exist.
I figure I have time to make errors in the first listing "not relevant"
before that becomes an actual issue.
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