Allow external package mappings in imports field in package.json

[sapphi-red]

It is allowed to write package names in imports field mappings like:

{
  "imports": {
    "#foo": "different-package"
  }
}

But this was not allowed by the schema (only strings with ./ prefix was allowed).

This PR fixes that.

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[madskristensen]

Thanks