Replace IO::All and Path::Class with Path::Tiny.

This should make dealing with paths somewhat more straight-
forward and reusable.

I have tested correct operation (both happy and unhappy paths) for the following commands:

- duckpan check
- duckpan query
- duckpan server
- duckpan help
- duckpan test
- duckpan installdeps
- duckpan reinstall

`duckpan publisher` fails with an [apparently unrelated problem](https://github.com/duckduckgo/duckduckgo-publisher/issues/118).

I did not manually verify the `duckpan env` bits because they are covered with tests.