Allow user to provide a custom module resolution

[rdsedmundo]

The best solution would be to allow the user to provide an arbitrary module with a known interface, so the resolution can be completely customized instead of just supporting something like path aliases—although I'm not sure how this would plug-in with Rust, so perhaps starting with path aliases would be easier.

Related to #1555.

Example use case: allow @ to resolve to the closest package.json, so we can have absolute path resolution across all monorepo packages.