Cannot run autorest from anywhere but the autorest.swift root

[tjprescott]

Due to a limitation on CI, I wanted to run autorest from the swagger location like so:

autorest --input-file=swagger.json 
         --output-folder=<full_dest_path>
         --namespace=namespace
         --use=<full_path_to_autorest_swift>

This should work, but instead if fails:

[26.35 s] Writing Outputs.
  Error: ENOENT: no such file or directory, open '<full_dest_path>/Package.swift'
[26.49 s] Shutting Down.