Library Sync conflicts with imports.

[connorhaugh]

On an openedx platform with async workers enabled,

If a course author uploads a course which references some library that exists already on the platform, they run into errors.

This is because xml_importer makes an async call to spawn the import of the content from the library

And then it proceeds to attempt to copy & publish the library content block's children, which leads to the likely state where the celery task has not copied in the LCB's new children, and this means that the publish cannot see the children, and fails.

In order to fix this, I am making the call to sync_from_library an apply call, not a delay call, so that it operates synchronously until we figure this out.

[kdmccormick]

Seems like #34030 fixed the immediate issue.

I think it makes sense to keep this synchronous, since we need the content to be copied from the library before the course can be published. If this causes issues, then we can look into making publish and copy-from-library asynchronous, with publish blocking on the completion of copy-from-library.