Bug 20686

The "logical" metamodel should be a layer over the physical metamodel and
classpath, which would contain roughly the same classes as the physical one, but
directly associated with each other instead of using symbolic refs.
There has to be a way how to denote or express an association to a non-existent
entity, or at least a way how to put that information into the lower layer.
The metamodel may be split into parts, for performance reasons:
1. metamodel, which would bind together physical metamodel instances according
to the classpath hiding rules and leaves space for broken links. This part must
be persistently stored. Ideally, there will be only associations in this part.
2. metamodel, which would contain instances for convenient access of most users,
but is a candidate for being transient and being derived from the (1) and the
physical/classpath metamodels.