Fix part I of T53977: Severe problem with multiple instances of a library (save and...

Fix part I of T53977: Severe problem with multiple instances of a library (save and reload).

The issue was that when a same lib was found several times in loaded
.blend, we'd only keep the first occurence. But since Blender expects
next data-blocks to belong to last found library, we could actually
be adding data-blocks assigned to copies of the duplicated lib to
another, totally unrelated lib.

Those data-blocks were then obviously not found when actually loading
libs content, and lost.

Note that this only fix one part of the issue, current code can
generate several copies of same linked data-block now, will fix in
another commit.