cacheddir: disable cache (avoid memory leak)

The global dircache is no longer supported, since it could
be a memory leak for API consumers. Any cacheddir callers
should use higher-level caches instead, when necessary.
util.listir: simplify and optimize

Use zip() to simplify, and fix "list = list + foo" usage to use
list.extend() or equivalent.
getconfig: share recursion code with grablines
RepoConfigLoader: support repos.conf directory