title: Using label path to check if file location exists
body: <p>Is there an easy way to get hold of a path object so I can check if a given label path exists. Say for example <code>if path.exists("@external_project_name//:filethatmightexist.txt"):</code>. I can see that the repository context has this. But I need to have a wrapping repository rule. Is it possible to do this in a macro or Skylark native call instead?</p>