Create a synthetic Bazel workspace.

It's under out/soong/workspace and is a symlink forest that merges BUILD
files generated by bp2build and the source tree.

bazel-* symlinks and $OUTDIR are currently ignored.

Test: Presubmits.
Change-Id: If14df23bd0f6a6f2d7d1140f95213f2aee66d652
diff --git a/bp2build/Android.bp b/bp2build/Android.bp
index d2a8729..abd79f5 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -14,6 +14,7 @@
         "constants.go",
         "conversion.go",
         "metrics.go",
+        "symlink_forest.go",
     ],
     deps: [
         "soong-android",