[infogulch]

This can go even further.

All the go files in a directory have to declare the same package name (`package mypkg`), with one exception: a test file (filename ends with `_test.go`) that has a package name that's the same as the normal package but ends with `_test` (as in `package mypkg_test`) is allowed to be in the same directory.

These are actually completely separate packages and you have to import the main package if you want to actually use it. But when you run `go test` it automatically complies and runs tests in both packages. This means that you can import both your package and a dependent package to make sure interfaces are satisfied without any circular dependencies.