[xylon] Legacy Libraries DEPR: Preserve legacy library defaults

[kdmccormick]

We want course references to legacy library content to work, forever. The content will not be able to be updated as of Willow, and it will not be able to be migrated as of Xylon, but it should continue working from a student's perspective in all future releases.

LegacyLibraryContentBlock children currently load their "defaults" from legacy libraries at runtime. That means that if we delete legacy libraries, things like library-set titles will disappear from the blocks.

So, before removing legacy libraries storage support from modulestore, we should figure out a simple, thin way to ensure that courses can continue reading these "defaults", even if the defaults can never change going forward.

[kdmccormick]

@ormsbee fyi, a thing I just thought of which I think we'll want to address in the early 2027 timeframe