cmd_install chose the memory.provider path with _is_memory_provider_dir, a
substring search for "MemoryProvider" in __init__.py. A hook plugin whose
docstring says "Not a MemoryProvider" passed, and --enable replaced the
user's memory.provider with a name load_memory_provider returns None for
(exit 0). _defines_memory_provider parses the package's modules (no import)
for what _load_provider_from_dir can load: a register_memory_provider(...)
call or a MemoryProvider subclass.
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