LD64Dependency is needed on macOS only

Move ld64_dependency.rb to os/mac/.

Library/Homebrew/dependency_collector.rb

@@ -1,6 +1,5 @@
 require "dependency"
 require "dependencies"
-require "ld64_dependency"
 require "requirement"
 require "requirements"
 require "set"
@@ -61,9 +60,7 @@ class DependencyCollector
     Dependency.new("xz", tags)
   end
 
-  def ld64_dep_if_needed(*)
-    LD64Dependency.new
-  end
+  def ld64_dep_if_needed(*); end
 
   def self.tar_needs_xz_dependency?
     !new.xz_dep_if_needed([]).nil?

Library/Homebrew/extend/os/mac/dependency_collector.rb

@@ -1,3 +1,5 @@
+require "os/mac/ld64_dependency"
+
 class DependencyCollector
   def cvs_dep_if_needed(tags)
     return if MacOS.version < :lion