Rename our current module caching terminology to something more descriptive

[tritao]

Right now we use the term Module for the parser cache:

pub struct ModuleCacheKey {
    pub path: Arc<PathBuf>,
    pub include_tests: bool,
}

After some discussion with @IGI-111, we realized this could probably use a more descriptive name, maybe Package cache.

cc @JoshuaBatty

[JoshuaBatty]

Package is a more apt description for what is being cached than Module. Happy for this change to go ahead.