The `expand` callback is 'trivial' to replace, since it is only iterating
over ID pointers and calling a callback.
The only change in behavior here is that some pointers that were not
processed previously will now be.
In practice this is not expected to have any real effect (usually
the IDs used by these pointers would have been expanded through other
usages anyway). But it may solve a few corner cases, undocumented issues
though.
Part of implementing #105134: Removal of readfile's lib_link & expand code.
8bb5916183