Refactors how the publish registry is computed

[arcanis]

This diff:

• Updates the documentation to properly reference the split between npmScopes and npmRegistries
• Updates yarn npm login to properly store the registry credentials inside npmRegistries
• Makes yarn npm login and yarn npm whoami compatible with the publish registry

In concrete terms, the main difference is that the npmConfigUtils module now exports tools to let the caller of npmHttpUtils have slightly more control on which registry is meant to be used for a specific query. This is optional, and passing an Ident instead of a registry will go into the typical workflow.

[deini]

Do we need to update the npmScopes type then to not have the authToken?

[arcanis]

Yep, that sounds the right thing to do 👍