A future thing.
https://go.dev/blog/using-go-modules
and
https://www.youtube.com/watch?v=9cV1KESTJRc
Both helped. But also:
$ mkdir cto-game-go
$ cd cto-game-go
$ go mod init github.com/cto-game-go
Add the tcell dependency
$ go get github.com/gdamore/tcell/v2
Then copy and paste an example from tcell and away you go.
Module and package management in go considered interesting.