report-fossil: Parse arguments before trying to detect drivers

This fixes ./report-fossil.py --help, which previously just croaked with an error about not being able to detect the driver, because no .csv files were specified on the command line. Which isn't a great error message when you're...trying to figure out the command line.

While at it, we also drop the requirement that filenames must end with .csv. The code to check for that was basically to try and distinguish filenames from other arguments...which we don't need to do now that we properly do argument parsing beforehand. They still have to be .csv files, of course, but mandating extensions is a bit unusual.