rfctr(auto): add _PartitionerLoader (#3418)
**Summary**
Replace conditional explicit import of partitioner modules in
`.partition.auto` with the new `_PartitionerLoader` class. This avoids
unbound variable warnings and is much less noisy.
`_PartitionerLoader` makes use of the new `FileType` property
`.importable_package_dependencies` to determine whether all required
packages are importable before dispatching the file to its partitioner.
It uses `FileType.extra_name` to form a helpful error message when a
dependency is not installed, so the caller knows which `pip install`
extra to specify to remedy the error.
`PartitionerLoader` uses the `FileType` properties
`.partitioner_module_qname` and `partitioner_function_name` to load
the partitioner once its dependencies are verified. Loaded partitioners
are cached with module lifetime scope for efficiency.
