pw_build: pw_facade require_link_deps arg

- Make the pw_build_LINK_DEPS check in pw_assert a generic feature in
  pw_facade.
- Use "impl" instead of "deps" for the pw_assert dependencies and
  restructure the impl / backend split.

Change-Id: I75c0f7e67b3b97bfe333760897223ab4601649c0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/43980
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
diff --git a/pw_assert_log/BUILD.gn b/pw_assert_log/BUILD.gn
index f6726b0..46ce4c8 100644
--- a/pw_assert_log/BUILD.gn
+++ b/pw_assert_log/BUILD.gn
@@ -46,9 +46,10 @@
   sources = [ "assert_log.cc" ]
 }
 
+# TODO(pwbug/372): Reorganize this.
 # pw_assert_log doesn't have deps with potential circular dependencies, so
 # "deps" can be empty.
-group("deps") {
+group("pw_assert_log.impl") {
 }
 
 pw_doc_group("docs") {