Add ModEntrypoint as an API for AdapterLoadableClassEntry

[AlexIIL]

Since the only public changes are generics, existing mods which use getEntrypoints() (which they shouldn't as it's all either PLUGIN_API or LEGACY_EXPOSED) will continue to work in binary (compiled) form, but not source form.

ModEntrypoint intentionally doesn't expose the adapter or value at the moment since only loader itself should need it, at least in theory.

This also removes the plugins field from the QMJ reader, since it used to use AdapterLoadableClassEntry but isn't actually hooked up anywhere (as actual plugins don't get defined in this way)

This also has two extra API changes:

• Adds ModLicense.fromIdentifier & fromIdentifierOrDefault, which exposes the same methods in ModLicenseImpl
• Adds QuiltPluginManager.loadZipNow, which is an immediate version of loadZip

[AlexIIL]

This will go in 0.22, since it's a bit more breaking than other changes.