Revisit canonical_name usage

Given that we support now compilation unit with multiple contracts with the same name, we might need to revisit
https://github.com/crytic/slither/blob/4c976d5af56219eeef079e03a35009af3e03644d/slither/core/declarations/function_contract.py#L24-L31

And similar definition of `canonical_name`, as it might create issues.

One solution would be to use the source mapping definition of the contract to ensure uniqueness.

Related: https://github.com/crytic/slither/pull/1535#discussion_r1083792395