Function resolve_specifier 

pub fn resolve_specifier(
    files: &FileAccess,
    from: &FilePath,
    specifier: &str,
) -> Option<FilePath>

Resolve specifier, written in from, to a file inside the project root.

None when nothing readable answers it, which is an ordinary result rather than a failure: the type answer that needed it is then undefined and the importing file is incomplete.