go.mod: Root of Go Module Identity

A go.mod file anchors a Go module, declaring its canonical path and dependencies to turn a package directory into a versioned unit. Every Go project needs one at its root, and the module path inside becomes the prefix for every package path the module contains. That path also tells the go command where to download the code, from version control or through a proxy. The biggest footgun is releasing major version 2 or higher without the required /v2 suffix in the module path, which breaks the go command's ability to locate it.