Licenses are still special because they need to go down to the
license_kind, but other metadata (self-contained) providers can be
gathered generically.  This PR has two parallel implementations so we
can explore the ramifactions of each.

The early attempt was to make PackageInfo a first class element, equal
to LicensesInfo in the way we recursively gather it.  The problem with
that approach is that it requires you to scale licenses_core.bzl with
each new type.

A different approach is to have a generic TransitiveMetadataInfo
collector that can hold many different types of provider in it,
as long as they all obey the same protocol for identifying their
type. We pass a provider list to gather_licenses_info_common()
to select what to include.

This way, a user adding a private metadata type only needs to
add a new write-json capability for their custom provider.
That is still a little ugly, but at least we can share
gather_licenses_info_common().