dependency_collector: Make ant_dep and xz_dep public

They were already public on macOS, but they were made
private by mistake on all other platforms.
DependencyCollector.tar_needs_xz_dependency? depends
on xz_dep being public, so there's no turning back now :(

Library/Homebrew/dependency_collector.rb

@@ -61,6 +61,14 @@ class DependencyCollector
     parse_spec(spec, Array(tags))
   end
 
+  def ant_dep(tags)
+    Dependency.new("ant", tags)
+  end
+
+  def xz_dep(tags)
+    Dependency.new("xz", tags)
+  end
+
   def self.tar_needs_xz_dependency?
     !new.xz_dep([]).nil?
   end
@@ -138,14 +146,6 @@ class DependencyCollector
     spec.new(tags)
   end
 
-  def ant_dep(tags)
-    Dependency.new("ant", tags)
-  end
-
-  def xz_dep(tags)
-    Dependency.new("xz", tags)
-  end
-
   def resource_dep(spec, tags)
     tags << :build
     strategy = spec.download_strategy