The Project class needs something to run scripts and programs, and up to
this point that has been the ScriptRunner class from
pytest-console-scripts. But that makes every single use of Project
dependent on the a script_runner fixture - unless we decide to start
constructing custom ScriptRunner instances, but that would be a little
awkward since that class expects to be given things like option values
for the console-scripts plugin. It would be much nicer if we had
the option to swap out the runner and potentially use something simpler.

This commit makes that change by defining a new protocol type which
describes exactly the behavior we need for a runner. That behavior is
provided by ScriptRunner.run() (or a simple wrapper around it, if using
an old version of pytest-console-scripts where the signature is
different), but now we can use other implementations as well.

In addition, this commit also defines a new protocol for the return type
of a runner. As with the runner protocol, the return type protocol is
compatible with the version from pytest-console-scripts but can also be
implemented with something custom.

This commit encapsulates the determination of which version of
ScriptRunner.run() to use in a new fixture.

This commit changes the Project class so that we provide a runner to
the run() or run_cli() method, rather than to the constructor. This
means we can now create a Project without a runner, and only provide one
as needed in tests which actually use that functionality. In particular,
the live distribution package tests have no more need to accept a runner
at all.