Use dot-separated name resolution for relationship target

Use dot-separated name resolution for relationship target

The string argument accepted as the first positional argument by the
:func:`.relationship` function when using the Declarative API is no longer
interpreted using the Python ``eval()`` function; instead, the name is dot
separated and the names are looked up directly in the name resolution
dictionary without treating the value as a Python expression.  However,
passing a string argument to the other :func:`.relationship` parameters
that necessarily must accept Python expressions will still use ``eval()``;
the documentation has been clarified to ensure that there is no ambiguity
that this is in use.

Fixes: #5238
Change-Id: Id802f403190adfab0ca034afe2214ba10fd9cfbb