Refactor postgresImportForeignSchema to avoid code duplication.
Avoid repeating fragments of the query we're building, along the
same lines as recent cleanup in pg_dump. I got annoyed about this
because
aa769f80e broke my pending patch to change postgres_fdw's
collation handling, due to each of us having incompletely done
this same refactoring. Let's finish that job in hopes of having
a more stable base.