使用模块跟踪依赖关系的Go项目的软件包布局？

Go now provides modules for dependency management, and I'm a little bit confused as to how I should organize my projects.

In traditional $GOPATH mode, I would organize an application as follows:

├─ cmd/
|  └─ myapp/
|     └─ main.go
├─ otherstuff/
|  └─ file.go
└─ README.md, etc.

This is what I see most projects on GitHub doing.

However, now that we have modules, I'm not sure where to put go.mod. Does it go in the project's root directory? Does it go in cmd/[whatever]/? Should I still be putting main.go in the cmd/[whatever] directory or should it be in the project's root directory now?

From the wiki:

A module is a collection of related Go packages that are versioned together as a single unit. Most often, a single version-control repository corresponds exactly to a single module, but alternatively, a single version-control repository can hold multiple modules.

So putting go.mod right next to .git (or equivalent for some other VCS) is almost always the right thing to do. You would only create multiple modules in a single repository if the code in each module is truly independent from the other modules, so that the version of one module does not effect the other modules in any way.