Fix encoding problems in \input@path

Fix encoding problems in \input@path

As of LaTeX2e 2018, characters are made active earlier, which results
in new expansion problems.

Following a suggestion of Markus Kohm (pc) and the TL mailing list [1],
we embrace non-ASCII input paths in \detokenize. This relies on e-tex,
but I think we can assume this is nowadays available everywhere.

[1] http://tug.org/pipermail/tex-live/2018-May/041691.html

Fixes: #11146