kpsewhich.tex

\documentclass{article}
\usepackage{luacode}

\begin{document}
\begin{luacode}
kpse.set_program_name("kpsewhich")
build_kpse_path = string.match(kpse.lookup("l3build.lua"),"(.*[/])")
tex.print(build_kpse_path)

local function build_require(s)
  tex.print(kpse.lookup("l3build-"..s..".lua", { path = build_kpse_path } ) )
end

build_require("help")
\end{luacode}
\end{document}