As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

- Rename plugintestutils to testing.

- Don't run the tests from bst-plugins-template. This imports
  buildstream.plugintestutils so will have to be disabled to get
  through CI. This can be re nabled once bst-plugins-template has been
  patched.

Pylint can't know that pytest's fixtures are used in a file and
therefore reports false positives. Silencing all those errors

Pylint doesn't play well with pytest fixtures, we therefore need to
silence this error.

We want external plugins to be able to make use of the core testing utils.
This commit exposes the basic utilities which are currently in use in
bst-external plugins. If necessary, more utilities could be exposed in the
future.

Moves the following files from tests/testutils/ to
buildstream/plugintestingutils/:
o runcli.py
o integration.py

As part of this, this commit makes the following changes to runcli.py
and integration.py:
o runcli.py: Fix linting errors
o runcli.py: Add user facing documentation
o Integration.py: Add user facing documentation

This commit also ensures that if we try to use the 'old' commands,
BuildStream will fail and instruct the user to use the new command.

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.

This is part of the work towards #672

This also fixes #177 - the problem here was solved simply
by passing the project directory to `cli.run(project=project...)`