feature: create packages() for MODFLOW 6 as a executable

[jdhughes-usgs]

Is your feature request related to a problem? Please describe.
There isn't a problem with create packages() but is relatively obscure.

Describe the solution you'd like
A create packages() executable like get-modflow. Options could be available to download definition files from specified releases or provide path to custom definition files.

Describe alternatives you've considered
None other than using the current implementation create packages().

Additional context
None