Stands for an entity from external system domain. An external system might be 'maven', 'gradle', 'eclipse' etc, i.e. any
framework or platform which defines project structure model in its own terms. That module might be represented in terms
of this interface implementations then.
It's assumed to be safe to use implementations of this interface at hash-based containers (i.e. they are expected to correctly
override
#equals(Object)
and
#hashCode()
.