pw_assert: Reorganize backends for new pattern

- Move all of the pw_assert_basic implementation into the
  pw_assert_basic.impl target.
- Enable the require_link_deps check for pw_assert.

Change-Id: Ie1f64cf0b7704a51710221d7ef02415c788a6235
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44100
Commit-Queue: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_assert/BUILD.gn b/pw_assert/BUILD.gn
index 7196831..258cd13 100644
--- a/pw_assert/BUILD.gn
+++ b/pw_assert/BUILD.gn
@@ -61,8 +61,7 @@
   ]
   public_deps = [ dir_pw_preprocessor ]
 
-  # TODO(pwbug/372): Update projects to properly list pw_assert:impl.
-  # require_link_deps = [ ":impl" ]
+  require_link_deps = [ ":impl" ]
 }
 
 # Provide "pw_assert/assert.h" in its own source set, so it can be used without