Re-enable alias for `xetex`

[muzimuzhi]

Since @AlphaZTX contributed package unimath-plain-xetex to CTAN and then TeX Live contained it, texdoc xetex now opens unimath-plain-xetex.pdf instead of xetex-reference.pdf.

$ texdoc -lM xetex | head -5
xetex   6.5 /usr/local/texlive/2022basic/texmf-dist/doc/xetex/unimath-plain-xetex/unimath-plain-xetex-doc.pdf       Package documentation
xetex   6.5 /usr/local/texlive/2022basic/texmf-dist/doc/xetex/xetexref/xetex-reference.pdf      The document itself
xetex   4.0 /usr/local/texlive/2022basic/texmf-dist/doc/xetex/base/XeTeX-notes.pdf      About XeTeX
xetex   4.0 /usr/local/texlive/2022basic/texmf-dist/doc/xetex/font-change-xetex/font-change-xetex.pdf   Package documentation
xetex   4.0 /usr/local/texlive/2022basic/texmf-dist/doc/xetex/xesearch/xesearch.pdf     Package documentation

In case there will be more packages having xetex in their names and alphabetically lower than xetex-reference, this PR (re)enables alias for xetex to give xetex-reference extra scores.

[wtsnjp]

LGTM. Thanks!