Since python 3.3, collections has been moved to collections.abc module.
For backwards compatibility, they continue to be visible in this module through Python 3.7.
Subsequently, they will be removed entirely.

See https://docs.python.org/3/library/collections.html

Fix python warnings: Use collections.abc instead collections

See merge request !883

Chandan Singh authored Sep 22, 2018 and Chandan Singh committed Oct 19, 2018

As discussed in
https://mail.gnome.org/archives/buildstream-list/2018-September/msg00064.html,
add `bst source-checkout` command. This will allow users to checkout
sources for a given target.

* _frontend/cli.py: Add source-checkout command
* _pipeline.py: Add assert_sources_cached() method
* _stream.py: Add source_checkout method, abstract out
  __check_location_writable() method that used to part of checkout()