Introduce new "source" command group

Following the message thread
https://mail.gnome.org/archives/buildstream-list/2018-November/msg00106.html,
implement a new command group called `source`. Move existing `track`,
`fetch`, and the recently added `source-checkout` commands under this
group.

For `track` and `fetch`, this is a BREAKING change, as the old commands
have been marked as obsolete. Using them will result in an error message
that refers people to use the new versions, like `bst source fetch`
instead of old `bst fetch`. `source-checkout` will now become
`source checkout` (the dash has turned into a space), and is not a
breaking change as it was added in the current development cycle.

Note that the functionality to hide commands from help output was added
only recently in Click, so the minimum version of Click that we now
require is 7.0.

Summary of changes:

* _frontend/cli.py: Add `source` command group, mark previous versions
  as obsolete and hide them from the help output.
* _frontend/complete.py: Fix completion for hidden commands.
* setup.py: Bump Click minimum version to 7.0.
* tests: Update to cope with the new command names.

Fixes #814.