Previously, the code would assert in `BLI_fopen` when the path is relative.
There are two main options to make the path absolute:
1. Always use the path of the current .blend file as base.
2. Is the path of the .blend file "owns" the current node as base. So when
the Import node is part of a linked node tree, the path of the linked .blend
file is used as base.
Both options are useful depending on the specific use-case, but the latter
seems more consistent with other places in Blender. For more advanced
functionality, we might need nodes that give the current .blend file path
and one node to join paths.
Pull Request: https://projects.blender.org/blender/blender/pulls/134794
234810f01d