fix(publish): follow all configs for registry auth check

[wraithgar]

The "do you have auth configured" only takes into consideration a hard-coded
"registry" config, which means that if you don't have auth configured for
the npm registry, but you do for the one you have tied to a scope elsewhere
in your npmrc, we would erroneously tell you that to add a token.

This uses the same method that the rest of the publish chain uses to determine
which registry it would be publishing to, then sees if you have auth for THAT registry.

Because that other function does a hard fallback to the npm registry, there is no more
need for the exception we throw if you do not have one configured. Also, the npm cli
already defaults that config item, so you can't even set it to a falsey value if you
wanted

References

Closes #2567

[wraithgar]

Found bugs while improving the tests around this. More commits incoming.