fix: resolve issue with unused analyzer reporting false positives

Earlier it sometimes could happen that the order in which the graph
traversal was happening could cause the issues with reporting some
functions as unused while these were in fact used within application.
This simplifies flow of the checker as well as it provide a more error
prone solution.

lib/mix_unused/analyzers/unused.ex

@@ -10,6 +10,8 @@ defmodule MixUnused.Analyzers.Unused do
   def analyze(data, possibly_uncalled) do
     graph = Graph.new(type: :directed)
 
+    uncalled_funcs = MapSet.new(possibly_uncalled, fn {mfa, _} -> mfa end)
+
     graph =
       for {m, calls} <- data,
           {mfa, %{caller: {f, a}}} <- calls,
@@ -18,30 +20,10 @@ defmodule MixUnused.Analyzers.Unused do
           Graph.add_edge(acc, {m, f, a}, mfa)
       end
 
-    called =
-      Graph.Reducers.Dfs.reduce(graph, MapSet.new(), fn v, acc ->
-        if v in acc do
-          {:halt, acc}
-        else
-          edges = Graph.edges(graph, v)
-
-          called? =
-            Enum.any?(edges, fn %{v1: caller} ->
-              caller in acc or
-                Enum.all?(possibly_uncalled, fn {mfa, _} ->
-                  mfa != caller
-                end)
-            end)
-
-          acc = if called?, do: MapSet.put(acc, v), else: acc
-
-          {:next, acc}
-        end
-      end)
-
     for {mfa, meta} = call <- possibly_uncalled,
         not Map.get(meta.doc_meta, :export, false),
-        mfa not in called,
+        reaching = Graph.reaching_neighbors(graph, [mfa]),
+        Enum.all?(reaching, fn caller -> caller in uncalled_funcs end),
         into: %{},
         do: call
   end