Provides access to the annotation processor factory path for a Java project.
This class should not be instantiated or subclassed.
The factory path is an ordered Map.
Containers are things like jar files or plugins, that contain one or more
annotation processor factories.
Manage a Map>, with reverse links so that it is possible to
efficiently find all T1s that have a particular T2 associated with them.
Access to the map is synchronized, so that it is possible to read and
write simultaneously from multiple threads.