Add `bst source-checkout` command

Description

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.

Changes proposed in this merge request:

• _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()

This merge request, when approved, will close issue/bug:

---