trouble searching for a package from its content

Dear Debian team,

I was searching for the package having '/usr/include/xlocale.h' and both
https://packages.debian.org/sid/amd64/libc6-dev/filelist
and
https://packages.debian.org/search?searchon=contents&keywords=xlocale.h&mode=path&suite=unstable&arch=amd64

give me to libc6-dev.

Running 'dpkg-deb -c libc6-dev_2.26-4_amd64.deb' with the binary obtain at
https://packages.debian.org/sid/amd64/libc6-dev/download
I don't see this file in it and neither on my local sid (also searching
directly in /var/lib/dpkg/info/*.list).
Am I doing wrong to search for? What can I do?

Thanks,
Patrice

ps: the 2 first pages do not indicate the package version that may help to
check in a more consistent way.