[FEATURE composable-computed-properties] remove dead code.
[FEATURE composable-computed-properties] Add tests

- CP implicit keys fix.
- expressions that differ in non-cp args
[FEATURE composable-computed-properties] 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`.