[roll pigweed] pw_span: Update BUILD.bazel files

- Rename //pw_span:pigweed_span to //pw_span:pw_span.
- Make the <span> polyfill private in Bazel.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99545

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 50b6f3784a24bf8..08e18e565058e06
Roller-URL: https://ci.chromium.org/b/8810065255005744369
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8810065255005744369
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2fbfdbec53b108612e91e55b368cfdf8f351bae7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/100198
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/pigweed b/pigweed
index 50b6f37..08e18e5 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 50b6f3784a24bf89948de1eb5c3a49d6288e5493
+Subproject commit 08e18e565058e06352c364a927af4eab438ed22a