Replace python -m bio2zarr vcf2zarr with python -m bio2zarr.vcf2zarr?

[jeromekelleher]

I.e., make each command a subpackage, and give those subpackages an __main__ definition. This seems like a more rational approach to organising things, but it does mean that encapsulating all the CLI functionality in the cli.py module as we do now (which has a lot of advantages) may not be possible.

[jeromekelleher]

Note: if we do, get rid of the note on installation.md linking to this issue.

[benjeffery]

I feel like the space is more natural than the . but it's not a strongly held opinion. Nice to have all the cli in one file too.