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>