Revert "pathname: SEPARATOR_PAT is no longer used"

This reverts commit 163f2f7.  Due to
a previous typo, this constant was not marked as `private`, so Bundler
was using it.

Author: nobu

pathname_builtin.rb

@@ -291,6 +291,18 @@ def inspect # :nodoc:
     "#<#{self.class}:#{@path}>"
   end
 
+  if File::ALT_SEPARATOR
+    # Separator list string.
+    separator_list = Regexp.quote "#{File::ALT_SEPARATOR}#{File::SEPARATOR}"
+    # Regexp that matches a separator.
+    SEPARATOR_PAT = /[#{separator_list}]/
+  else
+    separator_list = Regexp.quote File::SEPARATOR
+    SEPARATOR_PAT = /#{separator_list}/
+  end
+  SEPARATOR_PAT.freeze
+  deprecate_constant :SEPARATOR_PAT
+
   # Creates a full path, including any intermediate directories that don't yet
   # exist.
   #