This is the main aggregator for all implementation code shared between modules.
Each module contains one or more service interfaces,
exposed statically using "X_" prefixed classes.
Core modules will define their API interfaces, their static accessor,
and often a "plain-jre" default implementation which can be replaced by injection override.
There must be zero external dependencies in any core modules.
A low-performance, low-feature default is preferred over thick, dependency-ridden shared inherits.