pylint: Add support for XLTE

XLTE(*) follows the same top-level packaging structure like
wendelin.core does.

Registration is done via new `register_xpkg` function instead of just
loop so that pkgname argument is properly captured in created lambda and
does not change after going to next iteration.

(*) https://lab.nexedi.com/kirr/xlte

/reviewed-by @jerome
/reviewed-on !1762