Instead of downloading the sources optionally, always download them.
This will help to catch problems introduced in the build that prevent
the sources from downloading (see [1] for example).
Keep the --src option but mark it as deprecated.
Add a new --no-src option to explicitly prevent download of sources.
[1] https://gerrit-review.googlesource.com/#/c/74859/
Change-Id: I9b6a9e210f83fe6a5663011dc5c0daeaa2f7fe46