[autoform] Go through guess_field even when field class is specified using uicfg
guess_field does not only find the proper field class for some
attribute/relation, but also do some basic configuration depending on e.g.
constraints. Before this patch, if one was specifying explicitly the field class
using uicfg.autoform_field, this automatic configuration wasn't happening.
Change form.field_by_name and ff.guess_field so explicit class are also
automatically configured now.
Closes #14474840