Experiment: remove `unknownItem` flag

Simplifies loading unknown packages/releases.
Force-refresh catalog.complete from catalog.official.refresh.

Of course, if we were to simply set forceRefresh to true from where
catalog.official was calling catalog.complete.refresh, then we would
have looped inifinitely: initializing the catalog with a `versionsFrom`
field pointing to an unknown release would call
`catalog.official.refresh`, which would call `catalog.complete.refresh({
forceRefresh: true })`, which would call `catalog.official.refresh`
because the unknown release is still not found, and so on.

`catalog.official.refresh` doesn't do any of its actual work when called
reentrantly or when called during a `catalog.official.refresh` yield, so
it makes sense for `catalog.complete.refresh` to be part of that work;
in other words, `catalog.complete.refresh` shouldn't be called if
`catalog.official.refresh` is called reentrantly or during a yield, just
as we don't hit the server again under such conditions. And this also
prevents the infinite loop.

For reasons I don't fully understand, putting the
`catalog.complete.refresh` call before the futures are returned didn't
quite work. Turns out the refreshFutures aren't actually used anyway
(Future wasn't imported from this file; I have a sneaking suspicion the
refreshFutures thing hasn't actually been tested) so this is basically
dead code, and I replaced it with just making `catalog.official.refresh`
return immediately when called re-entrantly or during a yield.

Now we're successfully checking with the server before declaring that a
package/release doesn't exist, and not infinite looping if the
package/release really doesn't exist even on the server.
Add catalog.official._refreshFutures back in.

This allows `catalog.official.refresh` to be called during another
refresh that has yielded.

`catalog.official.refresh` still cannot be called re-entrantly, so
`recordOrRefresh` checks if a refresh is in progress before starting a
new one.
Merge branch 'packaging-experimental-unknown-item' into packaging