Modules (aka packages or bundles) are widespread in modern frameworks. Unfortunately each framework has its own convention and tools for writing them. The goal of container-interop and more specifically definition-interop is to help developers write modules that can work in any framework.
definition-interop contains interfaces helping developers describe container definitions (objects that can be cast to a container entry).
This package contains a set of test suites that can be used to ensure that a container/compiler is indeed compatible with definition-interop.
composer require --dev container-interop/definition-interop-tests:dev-master
This package contains a number of definition instances, and the associated PHPUnit tests that should match those instances.
Container / compilers compatible with container-interop should be able to pass any of those tests.
This package contains a AbstractDefinitionCompatibilityTest class. This is an abstract PHPUnit test class.
In your package, you should extend this class and implement the compileDefinitions method. This method should return a container-interop compatible container.