Attempt at reducing the possibility of having import cycles when user configs import rez

[JeanChristopheMorinPerso]

Fixes #1904

Attempt at reducing the possibility of having import cycles when user configs import rez.

A simple user config that simply imported rez.package_order would fail due to circular imports.

This is a big issue with rez where we already have to do a lot of local imports...

Anyway, this PR is just an experiment. It does remove the likelihood of cycles.

Another approach we could use could be something like https://rez.readthedocs.io/en/stable/configuring_rez.html#delay-load but instead of accepting a path, it could accept a function. That function could be safely called after the config is loaded. Not too sure how it would work or if it would even work.

From my analysis, rez.serialise was the most problematic. It's a core module that gets imported in multiple places. It should very much avoid importing modules like rez.config. In an ideal world, none of our "utilities" module should import rez.config.