XML: schemas.c: prepare the ground for unfold-transform-refold logic

Or more fittingly, for "transformation wrapped in the context
manager-like auxiliary pre-/post-process transformations
(dubbed enter/leave)".

Notably, add add_schema_by_version function to wrap add_schema, which
itself should not then be used directly when schema specified through
"version".

This commit should exceptionally be rather compiled with
CFLAGS=-Wno-error=unused-function, for apparent reason.