pw_span: Provide <span> through pw_polyfill

- Have pw_polyfill:overrides depend on pw_span.
- Remove explicit deps on pw_span.

Change-Id: Ibdd996d2764e737e487de5038cc84bd0f825c882
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/30043
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_checksum/BUILD.gn b/pw_checksum/BUILD.gn
index 048964c..d1209af 100644
--- a/pw_checksum/BUILD.gn
+++ b/pw_checksum/BUILD.gn
@@ -32,10 +32,7 @@
     "crc16_ccitt.cc",
     "crc32.cc",
   ]
-  public_deps = [
-    dir_pw_bytes,
-    dir_pw_span,
-  ]
+  public_deps = [ dir_pw_bytes ]
 }
 
 pw_test_group("tests") {