Function resolve_active 

pub fn resolve_active(
    tool: &str,
    start_dir: &Path,
    global_tools: &BTreeMap<String, String>,
    idiomatic_files: &[&str],
) -> Option<ActiveVersion>

Resolve the active spec for tool, walking up from start_dir. Global config pins are consulted last. idiomatic maps a tool to its idiomatic filenames.