Which package defines \smash?

The \smash command is active by default. No package needed.

% arara: pdflatex

\documentclass{article}
\begin{document}
xxx
\fbox{\rule{1cm}{2cm}}
\fbox{\rule[-1cm]{1cm}{2cm}}
\fbox{\smash{\rule{1cm}{2cm}}}
\fbox{\smash{\rule[-1cm]{1cm}{2cm}}}
\fbox{\raisebox{0pt}[0pt][0pt]{\rule[-1cm]{1cm}{2cm}}}
\fbox{\raisebox{-1cm}[0pt][0pt]{\rule{1cm}{2cm}}}
\end{document}

\smash is a macro defined in both plain TeX and the LaTeX kernel.

You don't need to load any package to use it.