modules: add a secondary go.mod to segregate CLI dependencies

Author: MichaelMure

cobra/go.mod

@@ -0,0 +1,11 @@
+module github.com/spf13/cobra/cobra
+
+go 1.12
+
+require (
+	github.com/mitchellh/go-homedir v1.1.0
+	github.com/spf13/cobra v0.0.0
+	github.com/spf13/viper v1.7.0
+)
+
+replace github.com/spf13/cobra => ../