This is how Go worked in the old days, before modules. Nowadays, the version constraints are specified in go.mod. Because the fully-qualified package names are

This is how Go worked in the old days, before modules. Nowadays, the version constraints are specified in go.mod. Because the fully-qualified package names are used to import them, you can reconstruct go.mod from the sources, assuming that you don’t care about version numbers. The source code says which packages, the go.mod file says which versions. (Major versions have different import paths.)