[#2780] Allow default concept compendiums to be defined

[arbron]

Takes advantage of the existing CONFIG.DND5E.sourcePacks object to define packs for backgrounds, classes, and races.

Closes #2780

[Fyorl]

It doesn't seem like these have a use beyond _onFindItem, and any changes users or modules make to these variables will stop working once we adjust _onFindItem to launch the compendium browser instead.

[arbron]

It doesn't seem like these have a use beyond _onFindItem, and any changes users or modules make to these variables will stop working once we adjust _onFindItem to launch the compendium browser instead.

Probably true, but it's a fairly minor change to fix an annoyance people are having. I doubt any proper modules will use it (we could even add a comment to discourage that), but I could see myself using it for now with my own private content module while waiting for the compendium browser to come along.