Revisit canonical_name usage

[montyly]

Given that we support now compilation unit with multiple contracts with the same name, we might need to revisit

slither/slither/core/declarations/function_contract.py

Lines 24 to 31 in 4c976d5

	if self._canonical_name is None:
	name, parameters, _ = self.signature
	self._canonical_name = (
	".".join([self.contract_declarer.name] + self._internal_scope + [name])
	+ "("
	+ ",".join(parameters)
	+ ")"
	)

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: #1535 (comment)

[0xalpharush]

Related: issues when contracts have the same name #1372