[roll third_party/pigweed] pw_protobuf: Move classes to the internal namespace

Codegen classes like MessageField should not be used directly, so move
them to the internal namespace.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/110230

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: eec77bd1b6df8a2..8e2a79e0bec7c18
Roller-URL: https://ci.chromium.org/b/8802914451261510913
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ief10cdb30823e90decbd8abcdf8f1020dea07f2d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/110520
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index eec77bd..8e2a79e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit eec77bd1b6df8a2879f4d5689af9b0974c709701
+Subproject commit 8e2a79e0bec7c18599970607b8291d889fa569bf