Audit formulæ for easy-install.pth files

Closes Homebrew/homebrew#32860.

Library/Homebrew/cmd/audit.rb

@@ -568,6 +568,7 @@ class FormulaAuditor
     audit_check_output(check_non_executables(f.sbin))
     audit_check_output(check_generic_executables(f.sbin))
     audit_check_output(check_shadowed_headers)
+    audit_check_output(check_easy_install_pth(f.lib))
   end
 
   def audit

Library/Homebrew/formula_cellar_checks.rb

@@ -120,6 +120,19 @@ module FormulaCellarChecks
      "The offending files are:  \n  #{files * "\n  "}"]
   end
 
+  def check_easy_install_pth lib
+    pth_found = Dir["#{lib}/python{2.7,3.4}/site-packages/easy-install.pth"].map { |f| File.dirname(f) }
+    return if pth_found.empty?
+
+    ["easy-install.pth files were found in #{pth_found.join(", ")}.",
+      <<-EOS.undent
+        These .pth files are likely to cause link conflicts. Please
+        invoke setup.py with options --single-version-externally-managed
+        --record=install.txt.
+      EOS
+    ]
+  end
+
   private
 
   def relative_glob(dir, pattern)