xmto: fix missing DTD/entities errors

`xmlto` forces the `--nonet` flag on `xsltproc`, which means it throws up an
I/O error whenever it needs to hit the network to pull down any files. So we
patch out the forced flag, and additionally add `docbook-xsl` as a dependency,
which'll reduce the odds of having to hit the network in the first place.

Fixes Homebrew/legacy-homebrew#21040.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/xmlto.rb

@@ -6,8 +6,15 @@ class Xmlto < Formula
   sha1 '5d1aecd59d519066f94b4591722767c4e41bdc0f'
 
   depends_on 'docbook'
+  depends_on 'docbook-xsl'
   depends_on 'gnu-getopt'
 
+  def patches
+    # xmlto forces --no-net on xsltproc, which causes it to fail when
+    # DTDs/entities aren't available locally.
+    DATA
+  end
+
   def install
     # GNU getopt is keg-only, so point configure to it
     ENV['GETOPT'] = Formula.factory('gnu-getopt').bin+"getopt"
@@ -19,3 +26,17 @@ def install
     system "make install"
   end
 end
+
+
+__END__
+--- xmlto-0.0.25/xmlto.in.orig
++++ xmlto-0.0.25/xmlto.in
+@@ -209,7 +209,7 @@
+ export VERBOSE
+
+ # Disable network entities
+-XSLTOPTS="$XSLTOPTS --nonet"
++#XSLTOPTS="$XSLTOPTS --nonet"
+
+ # The names parameter for the XSLT stylesheet
+ XSLTPARAMS=""