In !3424 (merged) I changed @reject_keys on Config.Loader to runtime deps, but that wasn't enough. I've been putting off this MR because I wanted to see if my other changes would fix it, but it doesn't seem like it.
Having Config depend on Endpoint, even as a runtime dep, is not ideal because is creates cycles. And a lot of stuff has a compile-dep on Config.
Since @reject_keys is just checking the keys for equality (with ==, literally), it's okay to let the compiler ignore them with Module.concat/1. This is the way the Elixir team recommends you do it, if necessary.
This change cuts down a cycle and shrinks the rest: https://www.diffchecker.com/n9Jf8Sln
