Call `require()` once only

When profilling CPU usage, we're seeing a significant amount of waste that occurs because of the lazy `require()` calls that are then called repeatedly even after first load. There is still a non-trivial cost in looking up cached objects from the require cache, including calculating up and slicing up the full path to the file. When doing heavy XML building work, this can become a considerable burden.

These changes simply shift all of the `require()` calls to the top so they are done once. In XMLNode.coffee they are lazily done in the constructor of the class, only the first time (by checking for `null`), so they are still just done once.