Implement new "Validation step" classes and their unit tests

[marcocastignoli]

Implement classes to validate a set of sources / setting before passing them to the compiler. For example in the case of Solidity, the class will use information in metadata.json to validate existing sources and check for missing sources. The Vyper validation class could run the integrity check on the existing files.

In general this classes can return an instance of the ️Compilation (#1822) objects.