Slither doesn't resolve references across compilation units

As discussed on [telegram](https://t.me/ETHSecurity/79291), it seems to me that Slither does not resolve references across compilation units. Here is a reproduction repo: https://github.com/hacker-DOM/reference-resolving. The tldr is multiple `Contract` instances will exist in `Slither.contracts` for each contract that spans multiple compilation units, and so a regular usage of the Api like finding a contract with a given name (if there's just one in my codebase) and looking for its derived contracts will report inaccurate results.