fix(scripts): prevent false positive phoenix_live_view plugin detection

[yordis]

• Downstream repos that do not depend on phoenix_live_view were getting Phoenix.LiveView.HTMLFormatter templated into .formatter.exs, which breaks every Format CI job with Formatter plugin Phoenix.LiveView.HTMLFormatter cannot be found.
• The trigger is any dependency that declares phoenix_live_view as an optional dep, since those declarations live inline in other packages' mix.lock entries and matched the unanchored pattern.
• Anchoring on the top-level lock key keeps plugin detection consistent with how import_deps detection already works.
• Seen in chore: sync files with beam-community/common-config ex_machina#569, which picked up polymorphic_embed.