add "optional requirements" to package.yaml

It would be nice if rez's package files supported the concept of "optional requirements" (i'm sure we can come up with a less oxymoronic name). These packages would be loaded if found successfully, but otherwise would not result in an error.

my current scenario is that I'm using rez to build MtoA (the Maya to Arnold translator), which has an optional plugin for shave and a haircut. We have shave for maya 2012 installed, but we don't yet have it for maya 2014. it would be nice if in the mtoa package.yaml file I could effectively say "load shave if you can", then in the build script (in this case it's scons) handle the logic if it is or is not there.

This feature is a nice analog to cmake's find_package, without the REQUIRED flag.