Add external registry support to authentication check

[privatenumber]

Fixes #361

[itaisteinherz]

Please fix the linting errors (usually can be done automatically by running npx xo --fix).

[sindresorhus]

Why do we have to pass --registry explicitly to npm whoami? Seems like npm should pick that up itself.

[privatenumber]

@sindresorhus
I wish it would but that's not the case, and unfortunately thewhoami doc doesn't document this behavior very well.

I dug through the source code to determine that whoami gets the default registry from the .npmrc file (aka npm config get registry). The registry defined in .npmrc is the registry where npm install pulls from.

For the purpose of publishing, the relevant registry is in package.json:publishConfig.registry. This is why we have to explicitly pass it in.

In my use-case, I have a private company-project that uses the official npm registry to install dependencies from, but we publish to an internal registry for others at the company to install our project from.

[sindresorhus]

Alright. Thanks for explaining.