swift234

Fix some bugs in the dominance-caching logic.

Most notably, the source caches did not respect dominance.  The
simplest solution was just to drop them in favor of the ordinary
caching system; this is unfortunate because it requires walking
over the path twice instead of exploiting the trie, but it's much
easier to make this work, especially in combination with the other
caching mechanisms at play.

This will be tested by later commits that enable lazy-loading of
local type data in various contexts.