Spanification: Represent frontier changes explicitly.

This change is purely refactorial and does not affect the behavior of
the spanification tool.

This patch improves the readability and maintainability of the
spanification tool by extracting frontier change information from the
graph edges.

This is clearer, because:
- There were previously part of the graph, but the edges were directed
  in a specific direction to avoid impacting the graph algorithm. This
  is not something explained and/or easy to understand.
- We were conflating the meaning of `include_directive` to instead store
  the `lhs_replacement`. It highlights the need for a dedicate format.

In follow-ups, I will target other special edges.

Bug: 356643982
Change-Id: I6c5233ea2b94b09ad69cc72132f0acfff2ef4187
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6190594
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Ali Hijazi <ahijazi@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411617}