[migration] Improve update of in-memory schema during 3.18 CWAttribute.defaultval change
We weren't updating the schema properly so a bunch of structures still
had the old rdef with 'String' as object. That caused failures to add
new attributes later in the migration as their default value would be a
Binary object but the schema would expect a String.
Closes #3683640
[migration] always rebuild infered relation
This was skipped for some bad reason (see 12ad88615a12 which
introduced the change).
Fix for #231956 in Yams is necessary to allow this cset: during a
migration, we want to always reinfer relations while allowing explicit
redefinition of an infered relation later. Else, we may run the
migration with missing parts of the schema (the one that should have
been infered).
Closes #3685463.