Manifest for the PackagesPlugin which collect and handle extension packages.
Extension packages are packages which declare a setuptools entry point : exopy_package_extension pointing towards a callable returning a lits of plugin manifest. At application start up, the entry point is inspected and all collected manifest are registered. They are unregistered during application closing (after calling the ClosedApp handlers). Each manifest can declare a priority between 0 and 100 which will be used when unregistering (lower values unregistered first).
Manifest of the Packages plugin.
Factory function for the PackagesPLugin