Readcode always set relative paths of indirectly linked libs relative to
*current* .blend file, not to the library using it.
But BKE_library_filepath_set was then setting them relative to their
parent library, breaking checking code (and saved files even :((( ).
It is hidden behind the --debug-io flag for now.
Idea is to try to catch broken libraries state in current Main before we
actually write the file on disk, should help catching and understanding
what happens in Spring corruption cases.
