buildMountArgs consults no registry and substitutes a RAMResource for
any mount it was not given, so the bare `{type}` state GenericResource
inherited restored a custom backend as an empty directory instead of
refusing. Every config-backed TypeScript resource already sets
needs_override for exactly this reason; a GenericResource always holds a
live accessor, so it always sets it too. copy() is unaffected either
way, since it passes the live resource through, and the flag is what
makes it do so.
python writes the key as well, where its own loader ignores it (that one
rebuilds the class from the registry) but a TypeScript reader of a
python snapshot does not.
Zecheng Zhang
984f79e2fe
