To be used for MBS-8736.

While this package is currently only used by script/dump-entities-sql.pl
to print SQL to stdout, it will eventually be used as part of MBS-8736
to create actual data dumps, which don't involving printing.

This builds up the related entities encountered into %path_ids and fetches
relationships just once at the very end. This is better because we don't end
up fetching the same relationships twice, and we can have our entity methods
skip IDs that were already fetched (which was previously not possible,
because the result of those functions sometimes depended on @link_path).

This is useful for when we are dumping the entirety of those tables
separately.

This dump will be useful for people wanting to set up a development
server with some data to use, but without having to perform a full
import of our entire DB (which takes a lot of time and space).

This doesn't even work where it's used, and can be functionally replaced
by just generating a JSON dump from the new sample data dumps.

Instead of copying custom configuration to config.user.sh, scripts can
set the MBS_ADMIN_CONFIG env var to point to the location of the custom
config (relative to admin/). This is safer in case multiple scripts
requiring custom configuration are executing concurrently.