Ask LuaLaTeX to ignore an active catcode previously written into aux file by pdflatex

I’m writing a package that loads babel and which users might run under either pdflatex or lualatex. When babel’s french option is loaded, ":" is made active; and pdflatex writes the following into the .aux file:

catcode `:active 

This is all fine for pdflatex. However, if a user (for some reason) decides to starting running with lualatex, the existing .aux file is read, and an error is reported when subsequent lines of the .aux file contain ":". For example,

newlabel{eqn:1}{{1}{1}}

results in

Undefined control sequence. 
<argument> r@eqn:
      1

One can just trash the .aux file and go again, but I’d like to prevent the message from appearing at all.

Is there a means of telling lualatex what to do with this active character (say, read it as ":")?

I’d thought of a construction such as

RequirePackage{iftex}
iflualatex
   RequirePackage{etoolbox}
   AtEndPreamble{catcode`:<someaction>}
fi

but I am not sure what someaction ought to be…

This MWE illustrates the problem in a single document:

% !TEX encoding = UTF-8 Unicode
documentclass{article}
usepackage[french]{babel}
begin{document}
Some English. foreignlanguage{french}{Cette théorie formera désormais une des branches les plus importantes de la physique générale}. More English.

begin{equation}label{eqn:1}
f(x) = Ax
end{equation}

end{document}

documentclass{article}
makeatletter
ifxdirectluaundefined
AtBeginDocument{immediatewrite@auxout{detokenize{%
 ifxdirectluaundefinedelseendinputfi}}}
fi
makeatother
usepackage[french]{babel}

begin{document}
Some English. foreignlanguage{french}{Cette théorie formera désormais une des branches les plus importantes de la physique générale}. More English.

begin{equation}label{eqn:1}
f(x) = Ax
end{equation}

end{document}

ifx directlua undefined else endinput fi