cmd/lncli: move commands and export

We want to export some of our CLI code to re-use in other projects. But
in Golang you cannot import code from a `main` package.
So we need to move the actual code into its own package and only have
the `func main()` in the `main` package.

.golangci.yml

@@ -259,8 +259,8 @@ issues:
         - forbidigo
         - godot
 
-    # Allow fmt.Printf() in lncli.
-    - path: cmd/lncli/*
+    # Allow fmt.Printf() in commands.
+    - path: cmd/commands/*
       linters:
         - forbidigo