Generate combined deps-info for all updatable modules.

Introduce a singleton apex rule to merge contents of individual
deps-info results into a single output file.

Bug: 149622332
Test: m
Change-Id: I4ab7e1a3527fead97a81a5a2cb0e1e93a429117c

Android.bp

@@ -492,6 +492,7 @@ bootstrap_go_package {
     srcs: [
         "apex/androidmk.go",
         "apex/apex.go",
+        "apex/apex_singleton.go",
         "apex/builder.go",
         "apex/key.go",
         "apex/prebuilt.go",