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
87a4c0d3a8