box: fix fkey creation together with new field names

If a complex (tuple) foreign key is added along with a new field name
that participates in that foreign key, the foreign key does not work
correctly. This happens because the new name of a local field is added
to new_space->def->dict only in ModifySpace::alter, while before that,
new_space->def->dict points to the old dictionary with old names (see
ModifySpace::alter_def). So when local_field_no is initialized earlier
in alter_space_do -> space_create -> tuple_constraint_fkey_init, it's
unable to find the new field number in the old dictionary.

Fix this by moving `new_def->dict = alter->old_space->def->dict;` from
ModifySpace::alter_def() to ModifySpace::alter(). Note that, as before,
we refer to the old dictionary from the new space (just put new names
into it), because that dict is referenced by existing tuple formats.

Closes #7652

NO_DOC=bugfix