Enable embedders to include Perfetto protos

This meant:
* Generating pbzero and pblite in the same paths,
  to avoid search path magic or having to specify
  "pbzero" or "pblite" in the include path.
* (From Primiano) Prevent generation of all Perfetto .py proto files to avoid naming conflicts.
* Adding the root of the perfetto generated files to the search path to
  let Perfetto protos include headers from other protos.


Change-Id: Ie90cbbf6cf8233d94abd324e3c07f15a0987d047

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 782f9a5..a64b365 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -28,7 +28,6 @@
     ":perfetto_tests",
     "src/ipc/protoc_plugin:ipc_plugin($host_toolchain)",
     "src/protozero/protoc_plugin($host_toolchain)",
-    "tools:protoc_helper",
     "tools/ftrace_proto_gen:ftrace_proto_gen",
     "tools/proto_to_cpp",
     "tools/trace_to_text",
@@ -41,6 +40,7 @@
       ":traced_probes",
       "src/ftrace_reader:ftrace_reader_integrationtests",
       "test/configs",
+      "tools:protoc_helper",
     ]
   }
 }
@@ -141,7 +141,10 @@
   }
 } else {
   group("lib") {
-    public_configs = [ "gn:public_config" ]
+    public_configs = [
+      "gn:public_config",
+      "protos:public_config",
+    ]
     deps = [
       "src/tracing",
     ]