[roll third_party/pigweed] pw_build: Subtarget aliases for Python packages

- Create top-level Python package substarget aliases when the Python
  package name matches the directory name. For example, this allows
  referring to "//foo:foo.tests" as "//foo:tests".
- Update and fix docs.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40180

third_party/pigweed Rolled-Commits: a25df5fad6f35ad..b85cda4630cab1f
Roller-URL: https://ci.chromium.org/b/8850048470763501040
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8850048470763501040
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iea8875287b1abf3f37483976b9096f9a895f927b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/40780
Reviewed-by: 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 a25df5f..b85cda4 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a25df5fad6f35ad227950577122faa10661a0df7
+Subproject commit b85cda4630cab1fcb247f3a9cf9ebb3f2ac9394e