Make 'xz' and 'cvs' dependencies conditional

Library/Homebrew/dependency_collector.rb

@@ -63,11 +63,11 @@ class DependencyCollector
   end
 
   def cvs_dep_if_needed(tags)
-    Dependency.new("cvs", tags)
+    Dependency.new("cvs", tags) unless which("cvs")
   end
 
   def xz_dep_if_needed(tags)
-    Dependency.new("xz", tags)
+    Dependency.new("xz", tags) unless which("xz")
   end
 
   def zip_dep_if_needed(tags)

Library/Homebrew/extend/os/dependency_collector.rb

@@ -1,3 +1,2 @@
 require "dependency_collector"
 require "extend/os/mac/dependency_collector" if OS.mac?
-require "extend/os/linux/dependency_collector" if OS.linux?

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

@@ -1,5 +0,0 @@
-class DependencyCollector
-  def xz_dep_if_needed(tags)
-    Dependency.new("xz", tags) unless which("xz")
-  end
-end