Build: Sub-Package field verification

In case you are creating a set of packages using the feature, then the Sub-Package field must exist and it cannot be empty.

In case you are creating one package with a single control file, then the Sub-Package field cannot exist.

The Sub-Package field is expected to have a valid comma separated list of sub-package names such as runtime, development, documentation, etc. The name is used in the filename so if you want to see "-dev" in your package name (instead of "-development") then make sure to use the short name.

Note that each name in the list of sub-package names must be unique or an error is generated and the process stopped.

One of the names in the list may be followed by an asterisk (*) to request wpkg not to include that name in the output filename. If two ore more asterisks are found, the process stops with an error.