[fostr] Move fuchsia.modular.internal back to public/

The fostr code-gen assumes that all code-gen'd files are in public/.

TEST=compiles

Change-Id: I9384dfd420b0cb7ffec067c431b005430ae3c30f
diff --git a/lib/fostr/fidl/fuchsia.modular.internal/BUILD.gn b/public/lib/fostr/fidl/fuchsia.modular.internal/BUILD.gn
similarity index 100%
rename from lib/fostr/fidl/fuchsia.modular.internal/BUILD.gn
rename to public/lib/fostr/fidl/fuchsia.modular.internal/BUILD.gn