Improve path handling for ensure_import_from

[morgante]

If I pass a path instead of a module name to ensure_import_from then it frequently fails to find the right location.

We should be able to use absolute path and relativizing to construct the right relative path to ensure an import from within my repo.

Ex.

`foo` as $mod where {
  $mod <: ensure_import_from(`'./utils/foo')
}

If I run this on apps/my.js it should insert the import as ../utils/foo.