language/node: make packname detection more robust

This fixes some edge cases where verbose output from a prepublish script
could break our npm pack package name detection code by only using the
last line of the output printed by npm itself containing the desired
package name.

Library/Homebrew/language/node.rb

@@ -11,8 +11,10 @@ module Language
       # directory, consequently breaking that assumption. We require a tarball
       # because npm install creates a "real" installation when fed a tarball.
       output = Utils.popen_read("npm pack").chomp
-      raise "npm failed to pack #{Dir.pwd}" unless $CHILD_STATUS.exitstatus.zero?
-      output
+      if !$CHILD_STATUS.exitstatus.zero? || output.lines.empty?
+        raise "npm failed to pack #{Dir.pwd}"
+      end
+      output.lines.last.chomp
     end
 
     def self.setup_npm_environment