Reimplement DepSet as a wrapper around a generic implementation

Implement depSet as a generic depsets implementation using reflection,
and then make DepSet a type-safe wrapper around it.  This will allow
additional wrappers for depsets that work with other types.  All of
this can be replaced with generics once Go supports them.

Test: depset_test.go
Change-Id: Id9df17bcc76f6c1545e7eb498f298066cf8a7679
diff --git a/android/Android.bp b/android/Android.bp
index 4bd272d..f8c1d55 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -23,7 +23,8 @@
         "csuite_config.go",
         "defaults.go",
         "defs.go",
-        "depset.go",
+        "depset_generic.go",
+        "depset_paths.go",
         "deptag.go",
         "expand.go",
         "filegroup.go",