partition_into_regions: CSI indexed VCFs have incorrect sequence names

[jeromekelleher]

The index names for CSI indexed VCFs must be derived from the index itself, because sequence names in an indexed VCF refer to observed sequences, not those that are listed in the header. The correct logic (I hope) is here:

https://github.com/jeromekelleher/bio2zarr/blob/880c3afee4465b4b94b921c815d436f3e4a78a46/bio2zarr/vcf_utils.py#L400

Some tests that should be straightforward to port to sgkit are here: https://github.com/jeromekelleher/bio2zarr/blob/880c3afee4465b4b94b921c815d436f3e4a78a46/tests/test_vcf_utils.py#L21