Forbid Path::exists

This function is simply wrong. As its own documentation says:

Warning: this method may be error-prone, consider using try_exists() instead!

We should use try_exists or std::fs::exists instead, and we should add Path::exists to disallowed-methods in clippy.toml.

Upstream have managed not to fix this situation and failed to deprecate this broken function, or even add it to the default clippy lints :-/. But we shouldn't have it anywhere in our codebase. (Not even in tests.)

Tagging this ticket as "Bug" because any use of Path::is_exists is an error handling bug.