Rename mk2rbc/cmd to make go install work properly

The standard go tooling assumes that a package's binary name should be
the name of the containing folder. Before this change, `go install` or
`go build` (without `-o`) names this binary `cmd`.

Test: treehugger
Change-Id: I45a4e97f4a0d59744d644e30636250ceefe3bd1c

mk2rbc/Android.bp

@@ -19,7 +19,7 @@ package {
 
 blueprint_go_binary {
     name: "mk2rbc",
-    srcs: ["cmd/mk2rbc.go"],
+    srcs: ["mk2rbc/mk2rbc.go"],
     deps: [
         "mk2rbc-lib",
         "androidmk-parser",