Auto merge of #3978 - dethoter:separated-credentials, r=alexcrichton

Move API token into the separate file.

Fix of #3748. BTW, it's not clear what to do with old config.
Should I add a check for old config and try to remove [repository.token] field from it every time user add a new token?
Or should I just prefer to use a token field from a new config over the old one?

diff --cc src/cargo/util/config.rs
Simple merge