Accommodate column-based naming conventions for pk constraint

Accommodate column-based naming conventions for pk constraint

Repaired / implemented support for primary key constraint naming
conventions that use column names/keys/etc as part of the convention. In
particular, this includes that the :class:`.PrimaryKeyConstraint` object
that's automatically associated with a :class:`.schema.Table` will update
its name as new primary key :class:`_schema.Column` objects are added to
the table and then to the constraint. Internal failure modes related to
this constraint construction process including no columns present, no name
present or blank name present are now accommodated.

Fixes: #5919
Change-Id: Ic2800b50f4a4cd5978bec48cefea0a2e198e0123