This affects the cache key version (updated to 7) and introduces
a dependency on `ujson` which is BSD licenced as of the version
locked in `requirements.txt`

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Returning only empty directories leads to inconsistencies when computing
a manifest by combining results from multiple list_relative_paths()
calls as done by the compose plugin.

I.e., the same directory may be empty in one dependency and non-empty in
another dependency.  The merged file list will still contain that
directory even though it's no longer empty.

This inconsistency causes problems when calculating differences between
manifests. Returning all directories fixes these inconsistencies.

This is a change in API behavior.

Most split rules already included the relevant directories themselves in
addition to the directory contents.  Add the missing bin, sbin, and
libexec directories.

This is required to fix tests with the following commit that changes
list_relative_paths() to return all directories.

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

They are too specific to be included by default

Recommendation is if you are building in Linux is to use the
ones begin used in the freedesktop-sdk project, for example

See #645

Also update tests to be consistent with this

In element, platform asks for host os and architecture to default to when
SandboxConfig is initialised.
This changes element cache keys so those have been updated in the tests.