Remove requirement of VERSION_PART as argument

[coordt]

Description

As the command-line interface matures, the fact that both the VERSION_PART and FILES are arguments, and that VERSION_PART is always required is makes less sense.

We are gonig to deprecate VERSION_PART as an argument over several versions and introduce a new CLI option: --part.

Here are the planned steps:

1. Merge the VERSION_PART and FILES arguments and detect the VERSION_PART based on if the first item of the arguments is in the part configuration.
2. Add a --part option
3. Begin warning that specifying the VERSION_PART via arguments is deprecated and will be dropped in version 1.0. Use --part instead.
4. Drop support of VERSION_PART in version 1.0