Since partitions can be foreign tables not only plain tables, but
logical replication only supports plain tables, we'd better check the
relkind of a partition after we find it.  (There was some discussion
of checking this when adding a partitioned table to a subscription;
but that would be inadequate since the troublesome partition could be
added later.)  Without this, the situation leads to a segfault or
assertion failure.

In passing, add a separate variable for the target Relation of
a cross-partition UPDATE; reusing partrel seemed mighty confusing
and error-prone.

Shi Yu and Tom Lane, per report from Ilya Gladyshev.  Back-patch
to v13 where logical replication into partitioned tables became
a thing.

Discussion: https://postgr.es/m/6b93e3748ba43298694f376ca8797279d7945e29.camel@gmail.com