This was tricky because of the caching, and because use_pagespec always
adds a dependency. That would have made year calendars depend on the whole
pagespec, which is overly broad. So I removed the caching, format_month,
and in format_year just look at %pagesources to see if month pages are
available.
In format_month, I make it always call use_pagespec, so each month calendar
gets the right dependency and any influcences added. This means a bit more
work, but the added work is fairly minimal, and presence dependencies
remove a *lot* of work it used to do.
(100% untested!)
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