feat!: use pathlib.Paths + streamline related checks

[jsstevenson]

• use pathlib.Path where appropriate rather than using raw strings
• make variable names a bit clearer + break up the path vars vs the corresponding handlers (previously they were being reassigned which makes for messiness when running type checkers)
• streamline a bunch of checks relating to whether a given output file is going to be created. Previously there were a lot of redundant variables and args being passed around. The signatures on some of these methods were getting pretty hairy otherwise.
• refactor tests to use temp files where available + consolidate inputs