49a7a66004 and immediately previous commits began installing
PostgreSQL 12 from their apt repository. On machines which already
have the distribution-provided version of PostgreSQL installed,
however, this leads to failure to apply puppet when restarting
PostgreSQL 12, as both attempt to claim the same port.
During installation, if we will be installing PostgreSQL, look for
other versions than what we will install, and abort if they are
found. This is safer than attempting to automatically uninstall or
reconfigure existing databases.
31f1f10501