Change how `etag` is stored for file download source plugins

We implemented etag recognition in #62 (closed), but we got it wrong.

The etag should be stored as a separate file, in the Source implementation's local state directory beside the tarball (or zip file or whatever) in quesion.

The etag is not project data, so it should not be in the project.

See this comment also: #62 (comment 69867332)