implement support for DEPEND_EXISTS

Preliminary support, anyway.

If a dependency only includes DEPEND_EXISTS, then only changes that
involved adding or deleting a page can trigger it.

This is complicated by internal pages, since the code did not previously
differentiate between add, delete, and change of internal pages.
Now it tracks change separately from add+delete, so DEPEND_EXISTS pagespecs
that actually match internal pages (which will probably be quite rare in
practice) should work.

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