calendar: rework so it can use use_pagespec

calendar: rework so it can use use_pagespec

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!)