Fix issue with imports not being removed

[larryk85]

CDT will compile smart contracts and include unused imports into the final smart contract.

To fix this the place to make changes will be in cdt-lld and revolve around how we determine liveness of those imports.

We are doing something wrong, so we will need to do a better analysis of liveness of those imports and prune those that are not used in the smart contract and those that are not reachable from 'apply' (the main entry point).