TUN-4063: Cleanup dependencies between packages.

- Move packages the provide generic functionality (such as config) from `cmd` subtree to top level.
- Remove all dependencies on `cmd` subtree from top level packages.
- Consolidate all code dealing with token generation and transfer to a single cohesive package.

token/launch_browser_unix.go

@@ -0,0 +1,11 @@
+//+build linux freebsd openbsd netbsd
+
+package token
+
+import (
+	"os/exec"
+)
+
+func getBrowserCmd(url string) *exec.Cmd {
+	return exec.Command("xdg-open", url)
+}