Stream files from fallback index chunk by chunk

[ddelange]

as of https://github.com/Pylons/pyramid/issues/2625 (v1.8), responses can be streamed.

after this PR, the lib:

• does a chunked read/write from the url to a tempfile in fetch_dist
• does a chunked read from the tempfile when streaming the response in download_package

[stevearc]

LGTM