Is there a way to mount or expose a folder within a package to another folder?
You can create an alias outside the bundle to expose this folder (by command-option dragging "Masters" out of the bundle). However, it is not always advisable to mess with the contents of a bundle, so proceed with caution. If you're just reading the photos, there's no problem, but Photos may not take kindly to modifications. The "safest" (but most annoying) way to access items in your Photos library is to export them from Photos itself.