[NICETHING beta] assert `needs` incompatibility with periods

needs isn't compatible with periods, because ultimately
you're going to need to call get('controllers.foo.bar')
(assuming needs: 'foo.bar'), which doesn't and can't
do what you want.

EAK solution is to slashify
global ember solution is to camelcase

Not sure if we can make a better error than just saying "don't use periods"