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.

This MR changes the behaviour of source checkout so that it is more
like artifact checkout. That is, a --directory option can be provided,
rather than a mandatory LOCATION.

In addition to this, the --tar option is no longer a boolean flag,
rather it expects a tarfile name (just like artifact checkout)

The appropriate tests have also been altered so that they support
the new API

This removes the --fetch option of bst source checkout, always enabling
fetch support. This is in line with fetching subprojects as needed.

- 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.

- Remove all wrong-import-order from pylint
- Order some subgroups of imports

Few different source cache tests have been to check that sources are
staged into the local CAS and fetched where appropriate

Part of #440