It is perfectly legitimate to create and/or use a method named `define`
in JS outside of RequireJS.
Unfortunately our requirejs.php wrapper is dumb and does not understand
this.
In the long term we need to stop doing this at all. We really should be
able to already, but every time I try to something prevents it.
In the interim, this change adds a secondary check to see if there is an
existing define which _does_ have the right name in it already.
This option was used to return the source files, but we do not support
this any more because we use source maps instead and require babel
transformation to support ES2015.
It does not make sense to support this code path any longer.
