make slither-flat work for top level errors, structs, enums

[smonicas]

fix #1410 #1837

• Fix the type hint for custom errors in SlitherCompilationUnit where they are CustomErrorTopLevel and not just CustomError
• Expose the custom error in SolidityCustomRevert so when analyzing a revert SOLIDITY_CALL in slithir it's possible to get the custom error used
• Set the logging level to INFO for the flattening tool otherwise the logs are not showed
• Changed list_top_level and list_contracts to be a set and not a list. This avoid duplicates and a necessay check before appending that the element is not present as this is error prone. For example at the moment this check is missing when adding a top level function so if you were to use a top level function n times you will get n definitions of that function in the flattened output