[roll third_party/pigweed] pw_software_update: Fixes for downstream use

The protos weren't quite building previously. This change fixes minor
.proto issues, streamlines directory names, and adds pw_software_update
to modules.gni so downstream projects can easily use it.

Testing: Successfully built downstream project dependent on protos.
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/58282

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: c4cc3cbc0450be8..a8a6f474fed9ed9
Roller-URL: https://ci.chromium.org/b/8837879765478860961
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8837879765478860961
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Icd71de75933bc5f5a54f767f59b8206a9671d235
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/58264
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 c4cc3cb..a8a6f47 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit c4cc3cbc0450be8b31282d89bfdff2eb6f5c97d4
+Subproject commit a8a6f474fed9ed905ae597476d8d1701500fe5f7