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
diff --git a/mk2rbc/Android.bp b/mk2rbc/Android.bp
index 4fa3eb6..cd80a4d 100644
--- a/mk2rbc/Android.bp
+++ b/mk2rbc/Android.bp
@@ -19,7 +19,7 @@
 
 blueprint_go_binary {
     name: "mk2rbc",
-    srcs: ["cmd/mk2rbc.go"],
+    srcs: ["mk2rbc/mk2rbc.go"],
     deps: [
         "mk2rbc-lib",
         "androidmk-parser",
diff --git a/mk2rbc/cmd/mk2rbc.go b/mk2rbc/mk2rbc/mk2rbc.go
similarity index 100%
rename from mk2rbc/cmd/mk2rbc.go
rename to mk2rbc/mk2rbc/mk2rbc.go