Add a clone method to ResultSource, switch obvious spots to it

Add a clone method to ResultSource, switch obvious spots to it

Not messing with the ::ResultSourceProxy::Table clusterfuck for now, too many
things can go wrong. Instead will explicitly instrument the callsites in
subsequent commits.

Also add assertions this does not get routed around: such use will throw from
here on out as long as one enables the necessary assert:

~$ DBIC_ASSERT_NO_ERRONEOUS_METAINSTANCE_USE=1 perl -Ilib -MDBIx::Class -e '
  bless ({}, "DBIx::Class::ResultSource")
'