Support prefix scopes

[possan]

Added support for prefixed scopes (trailing slash) Also made it use locally resolved files and not always return them as externals

Is this of any interest?

[trygve-lie]

Definitely interested in this :)

Just one small comment.

[tshelburne]

@trygve-lie What are the chances of getting this updated and merged? Just came across it, would be tremendously helpful for Deno build steps.