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>