[tools] Separate changing to contest directory and getting problems list

[mzuenni]

Will eventually close #466, which was a follow-up of #166

[mzuenni]

For now, redid the work from #166 on top of main. Old todo message from @mpsijm:

I was thinking of something like the following (but I'll need to try this to see how it works out):

1. In main():
   1. Read command line args (without passing the personal config as default values)
   2. Set initial_cwd = os.getcwd()
   3. cd to to contest directory (this reads --contest and --problem only from the command line, but having them in a personal config file wouldn't make sense anyway 😛)
   4. Read personal config
   5. Call run_parsed_arguments with combined args object
2. In run_parsed_arguments(args):
   1. if action in ['new_contest', 'new_problem']: check if --contest and/or --problem are set, and throw an error if so (only new_problem --contest would be valid). Then, os.chdir(initial_pwd) and continue with the action
   2. else: continue processing the args as usual

Maybe 2.i should be checked before 1.iv, but that requires a bit more rewriting. I'll experiment with it for a bit 🙂

[mzuenni]

@mpsijm is this what you had in mind? ^^'

[RagnarGrootKoerkamp]

some small comments but overall LGTM.
Didn't run/test anything myself.

[mpsijm]

This is definitively cleaner than how it was before, thanks! ❤️ The rewrite of config.args in #473 also helps 😄