Move `Library.runtime` to be a pointer, move the related
`LibraryRuntime` struct to `BKE_library.hh`. Similar to e.g.
Mesh.runtime, that pointer is expected to always be valid, and is
allocated at readtime or when creating a new Library ID.
Related smaller changes:
* Write code now uses standard ID writing codepath for Library IDs too.
* Runtime pointer is reset to nullptr before writing.
* Looking up a library by its absolute path is now handled through a
dedicated utils, `search_filepath_abs`, instead of using
`BLI_findstring`.
Pull Request: https://projects.blender.org/blender/blender/pulls/134188
This merge does not rename any ID. In case a conflict happens (an ID of
the same name and library already exists in destination Main), its
counterpart in source Main is not moved, and its usages by other
moved-from-source IDs are remapped accordingly.
Libraries are also properly de-duplicated, and remapped as necessary.
Source Main is always freed by this function, since it is either empty,
or contains left-over IDs very likely to have invalid relationships
after the merge.
NOTE: This commit also enables `main_test.cc` unittests, which was
comitted by mistake in fe3cb11ae4, sorry about that.
Pull Request: https://projects.blender.org/blender/blender/pulls/115671
