red_knot_project: sort diagnostics from checking files
Previously, we could iterate over files in an unspecified order (via
`HashSet` iteration) and we could accumulate diagnostics from files in
an unspecified order (via parallelism).
Here, we change the status quo so that diagnostics collected from files
are sorted after checking is complete. For now, we sort by severity
(with higher severity diagnostics appearing first) and then by
diagnostic ID to give a stable ordering.
I'm not sure if this is the best ordering.
