[roll third_party/pigweed] pw_build: Mirror all Python sources under one dir

- Create package_metadata.json files for each python Package in the GN
  build graph.
- New GN template to copy Python package sources under one directory.
- New GN target to copy all Pigweed's Python packages. This will
  operate on both in-tree and generated Python packages.
  pw_create_python_source_tree("build_pigweed_python_source_tree")

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56841

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 284b51470fe5bf2..211dce4c94f74b4
Roller-URL: https://ci.chromium.org/b/8837337319992234241
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8837337319992234241
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I0fb3ac7535f7be4dd1d046d2013e768247556cc6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/59160
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 284b514..211dce4 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 284b51470fe5bf288b6a4e095a62acbe0d504fe9
+Subproject commit 211dce4c94f74b43e942e1d461bc0e4f9df1b2ee