etc.install: handle recursive directory installs.

We need to install the helper module not just on `etc` but also on all
subdirectories of it too. Also, handle the case where we install
a subdirectory with etc.install.

Closes Homebrew/homebrew#26145.

Library/Homebrew/extend/pathname.rb

@@ -48,6 +48,7 @@ class Pathname
     dst = dst.to_s
 
     dst = yield(src, dst) if block_given?
+    return unless dst
 
     mkpath
 

Library/Homebrew/install_renamed.rb

@@ -2,6 +2,12 @@ module InstallRenamed
   def install_p src, new_basename = nil
     super do |src, dst|
       dst += "/#{File.basename(src)}" if File.directory? dst
+
+      if File.directory? src
+        Pathname.new(dst).install Dir["#{src}/*"]
+        next
+      end
+
       append_default_if_different(src, dst)
     end
   end
@@ -12,6 +18,14 @@ module InstallRenamed
     end
   end
 
+  def + path
+    super(path).extend(InstallRenamed)
+  end
+
+  def / path
+    super(path).extend(InstallRenamed)
+  end
+
   private
 
   def append_default_if_different src, dst