Allow physical checks to return multiple results
flake8 is currently inconsistent in how it handles the return results of checks. This PR amends
run_physical_checks in a backwards compatible way, so that multiple results can be returned by a single physical check.
run_logical_checks, multiple results can be returned.
run_physical_checks, only a single result can be returned.
My current use case is writing a plugin that runs one complex regex over each physical line. Depending on what matches, I'd like to return different error codes and descriptions. There may be more than one match per line; at the moment, I have to split this out into multiple plugin entrypoints, one for each possible return type.