Function cargo::ops::cargo_install::installed_exact_package

fn installed_exact_package<T>(
    dep: Dependency,
    source: &mut T,
    config: &Config,
    opts: &CompileOptions,
    root: &Filesystem,
    dst: &Path,
    force: bool
) -> CargoResult<Option<Package>>where
    T: Source,

Checks if vers can only be satisfied by exactly one version of a package in a registry, and it’s already installed. If this is the case, we can skip interacting with a registry to check if newer versions may be installable, as no newer version can exist.