Add interface for FilePath, DirectoryPath and Path.

By implementing these interfaces on `ConvertableDirectoryPath` and `ConvertableFilePath` usage of paths received from the `Directory(..)` and `File(..)` aliases will be much easier.

We should also change all internal methods from taking `FilePath`/`DirectoryPath` to `IFilePath`/`IDirectoryPath`.

Another benefit this brings is that we remove A LOT of implicit conversions like ConvertableFilePath -> DirectoryPath which is always nice.

Start by locating ConvertableDirectoryPath, ConvertableFilePath, FilePath, DirectoryPath, IFilePath, and IDirectoryPath, then trace the Directory(..) and File(..) aliases that produce these values. Done means the two convertable types implement the interfaces, internal methods accept the interface types, and the unnecessary implicit conversions are removed.

Written by the indexing model from the issue text.