Allow for import map with `github` scheme

[eturino]

With npm we could do an import to a particular repo and branch/tag/commit and will use ssh to get the code, which will take care of security concerns when building in the CI for example.

I would like to have this in deno.json

{
  "imports": {
    "somelib": "github:my-org/somelib#my-branch"
  }
}

This would allow us to work with unreleased versions of internal packages before they are released, to check that they are compatible and to allow for debugging and checking fixes in the deno apps using the somelib code.