Function install_required 

pub fn install_required(
    git: &dyn GitRunner,
    fetcher: &dyn Fetcher,
    builder: &dyn ComponentBuilder,
    spec: &RequiredSpec,
    cache_root: &Path,
    user_root: &Path,
    policy: RefreshPolicy,
) -> Install

Resolve, build and stage one required plugin.

Blocking. Run on spawn_blocking.