[FEATURE composable-computed-properties] Several CCP Fixes.

- Make CP implicit keys calculate correctly when they have no dependent keys.
- Remove some dead code.
- Test expressions that differ only in non-cp args.
- Support CCP depkey paths.

Dependent keys are used to form the implicit key for a CP, so here we replace
'.' to ensure the resulting key is not interpreted as a path by `Ember.get`.
Merge pull request #4182 from hjdivad/fix-ccp-bugs

Fix a bug with composable computed properties.