mdwtools.tex (\env, \package): Define using `\def', not `\let'.

This allows the commands to be used in PDFstrings, and hence in section
headings.  The `hyperref' machinery builds the PDFstring by making many
control sequences, such as `\textsf', be nops, but `\package' or `\env'
will still cause trouble if they're aliases of the old font-changing
commands.