Resolve corelib automatically

[wawel37]

Problem:

To test the rules, contributors have to provide path to corelib manually like this:

CORELIB_PATH="/path/to/corelib/src" cargo test

AC:

Make it so the corelib is resolved automatically, and if not found, display a proper error

[mkaput]

you might want to just copy-paste this: https://github.com/software-mansion/cairols/blob/eb6a1ae14dc0c448dfd88a6f7766930d1b08b756/src/project/unmanaged_core_crate.rs#L84