Replace backlink refs with weakrefs

[mrbean-bremen]

• includes links to filesystem, patcher and parent dir
• in most places the filesystem is assumed (asserted) to exist

This hopefully fixes #1267, though it also introduces potential crashes if a backlink is no longer available. While this should not happen, it cannot be excluded, so this needs proper testing.

Tasks

• Unit tests added that reproduce the issue or prove feature is working - n/a
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
• Unit tests passing
• For documentation changes: The Read the Docs preview builds and looks as expected