It's important to not only reference an external project (e.g. library) but also to reference a particular version of that library. (e.g. newer versions could remove deprecated methods which you are using, i.e. which weren't deprecated when you wrote your code.)

Different versions of your code could rely on different versions of the library (e.g. you update your code to a newer version of the library.) So which version of the library you rely on also needs to be version-controlled.