Refactor: use division operator to join paths (#11413)
Starting with `resolve_package_path` and its associated tests,
this refactoring seeks to make path concatenation more
readable and consistent within tests/functions.
As discussed in #11413:
- code is free to use either `/` and `joinpath`
- consistency within a function is more important than consistency across the codebase
- it is nice to use `/` when it is more readable
- it is nice to use `joinpath` when there is little context
- be mindful that `joinpath` may be clearer when joining multiple segments
