Rename `PackageStore::proto_path()` and `proto_vendor_path()` to something snappier.

[xfbs]

I still think that the "proto_path" is not related to the store, so:

proto/* -> the current package proto/vendor/* -> the "package store"

So if we would split out the logic for accessing the first path that would be great. I think the weirdness that the vendor dir lives inside of the proto dir creates these blurry borders