Cache monorepo package lookups

[jdb8]

I noticed that the rules call getPackages from get-monorepo-packages many times per execution, even when directory is the same (which I'd imagine it always is?). So just adding a module-level cache here saves us some work.

This isn't a huge time saving, but experimentally in a large monorepo I saw it shave off ~30s of total time on a ~2m run, which is non-negligible.

This PR assumes that it's safe to do this: if it's not safe, due to something I've overlooked, the alternative (better) approach is probably to add a config option allowing users to pass in a list of monorepo paths, so it doesn't need to be calculated at all by the plugin. Though if this is fine then it's less work since it doesn't require a new config option + documentation etc.