Bug#930495: My bad

[Michael Reed]

After reviewing a stretch system I see that I've mischaracterized the
issue.  Stretch has postgresql-9.6-postgis-2.3 and
postgresql-9.6-postgis-2.3-scripts which are analogous to buster's v11
packages, including being recommendations rather than depends.  My
notes don't say anything about me manually installing these but I must
have.

This is a weird situation though, with the postgis extension not being
in the postgis package, and the package instead having some utilities
only tangentially related to the extension.

Perhaps what is needed is a meta package (like postgresql) so someone
can install the extension once and have it upgraded across releases.
The stuff currently in the postgis package should be moved to an
optional postgis-utilities package (recommended by, but not a dependant
of the meta package).