lib: rename bt_resolve_field_path_context to bt_resolve_field_xref_context

lib: rename bt_resolve_field_path_context to bt_resolve_field_xref_context

In preparation to sharing some code with the field location resolving,
rename bt_resolve_field_path_context to bt_resolve_field_xref_context,
which will be used by both kinds of field cross-reference resolving.
Move it to its own file, which will be included by both.

Change-Id: Ia6b7e9fa9de76d16fa0244a66597c3a1bfb30271
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7303
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7327