texi: assume .texi.in generates .texi. This change refines the fix for https://bugs.gnu.org/54063. * bin/automake.in (scan_texinfo_file): if .texi doesn't exist, but .texi.in exists, read the latter for the Texinfo source. Use the @setfilename argument, if present, to generate rules. * t/txinfo-no-texi-but-texi-in.sh: new test. * t/list-of-tests.mk (handwritten_tests): add it. * doc/automake.texi (Texinfo): document this. * NEWS: mention this. (Doc changes written by Karl.)