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.

cmd/commands/devrpc_default.go

@@ -0,0 +1,11 @@
+//go:build !dev
+// +build !dev
+
+package commands
+
+import "github.com/urfave/cli"
+
+// devCommands will return nil for non-devrpc builds.
+func devCommands() []cli.Command {
+	return nil
+}