For includes specified with
\inputthat are not found directly, lilypond-book tries to determine the full path with the
kpsewhichutility. If this succeeds, the captured stdout of
kpsewhichhas a trailing newline appended, which has to be removed before opening the file.
Fall back to searching the include path if no
kpsewhichbinary is available.
Fix default path insertion for
Closes #6213 (closed).