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refactor: unify `preload_defaults` functions

Provide a global `Repo.preload_defaults` to load default preloads for a
given module, instead of using a distinct function for each one.

`Repo.preload_defaults`'s logic is to make modules define their own
`default_preloads/0` function, and use its return value as
`Repo.preload/3`'s second parameter.

Signed-off-by: Francesco Noacco <francesco.noacco@secomind.com>