reorganize Mapped[] super outside of MapperProperty

reorganize Mapped[] super outside of MapperProperty

We made all the MapperProperty classes a subclass
of Mapped[] to allow declarative mappings to name
Mapped[] on the left side.  this was cheating a bit because
MapperProperty is not actually a descriptor, and the mapping
process replaces the object with InstrumentedAttribute at
mapping time, which is the actual Mapped[] descriptor.

But now in I6929f3da6e441cad92285e7309030a9bac4e429d we
are considering making the "cheating" a little more extensive
by putting DynamicMapped / WriteOnlyMapped in Relationship's
hierarchy, which need a flat out "type: ignore" to work.

Instead of pushing more cheats into the core classes, move
out the "Declarative"-facing versions of these classes to be
typing only: Relationship, Composite, Synonym, and MappedSQLExpression
added for ColumnProperty.  Keep the internals expressed on the
old names, RelationshipProperty, CompositeProperty, SynonymProperty,
ColumnProprerty, which will remain "pure" with fully correct typing.
then have the typing only endpoints be where the "cheating"
and "type: ignores" have to happen, so that these are more or less
slightly better forms of "Any".

Change-Id: Ied7cc11196c9204da6851f49593d1b1fd2ef8ad8