[META] Introduce concept of internal resource types

Broke this out into subtasks because the patch was becoming unreadable. They can be committed independently or in a final combined patch.

Upgraded the task to Major because that's more accurate.

I'd put it around 77.77778% finished.

• 2 of the largest issues (isInternal and related resource types) are committed. That's 2/5ths of the meta-issue, but half the battle. [50% of effort/0.00000% remaining]
• Of the 3 remaining, one is nearly RTBC with only nits remaining (internal resource inclusion) [2.22222% remaining]
• One will be a very small clean-up issue (links to internal) [4.99% remaining]
• Only one is actively being worked on/discussed in detail (providing relationship routes) [15.01% remaining]

These are just some rough estimates, so please don't quote me on it.