Mojang is prepended in code as the first, identity source, and the
config templates say not to list it. Nothing enforced that. A listed
tag = "mojang" loaded, and nano's startup list printed it as if Mojang
had been pointed at that url, while the real Mojang was still asked
first. The listed entry was a separate third-party source: asked again
on every login that got past Mojang, adding up to five seconds when its
url was Mojang's own and it answered 204 each time.
Any case of "mojang" is now rejected at load with a message saying
Mojang is built in and must not be listed. The duplicate-tag check could
not catch this because the built-in source never passes through it.
The new test loads "mojang" and "Mojang" through LoadNano; both loaded
before this change.