avoid unnecessarily rebuilding pages with complex conditionals

I noticed that ikiwiki/formatting was beilg rebuilt when any page changed.
This turned out to be because it contained a complex conditional
"enabled(foo) or enabled(bar)", and the conditional plugin did not notice
that this consisted only of enabled() tests, and copied it unchanged into
add_depends. Thus, the page's dependencies were satisfied by any page
change.

The fix is to beef up the parser so that it can handle that and more
complex conditionals, and detect if they consist only of such tests.