Add flag to indicate if a package is a top level selectable package

[rgwilton]

Considering version selection, should the YANG library package definition have a flag to indicate whether a particular package can be selected by a client?

Probably the answer here is to defer this issue to a separate version selection draft that can add a flag via augmentation; or provide a separate list of packages that are selectable.

[rgwilton]

Alternatively, rather than flags, have separate lists that reference the top level packages may be a better solution.

[rgwilton]

With the current schema-selection model, having a top level flag is no longer required (or desirable)