Include files included by class file in fileset

[PHPirates]

Fixes #1053

To test this PR:

main.tex:

\documentclass{class}
\begin{document}
  \begin{align*}
    \pi
  \end{align*}
\end{document}

class.cls:

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{class}[2019/10/31]
\LoadClass{article}
\RequirePackage{style}
\endinput

style.sty:

\ProvidesPackage{style}[2019/10/31]
\RequirePackage{amsmath}
\endinput

Then check that there is no error in main.tex, but when commenting out the \RequirePackage{amsmath} then the error appears.