I just faced this issue today when working on #2594631: Migration integration. I had almost the same solution in mind before I found this issue. Since this is three months old, I hope you don't mind me making significant changes. Basically, I removed what I thought were out of scope changes and added a few more things like ability to specify the property name where field_type will be available in a source plugin.
Interestingly, the D6 version of field formatter migration already uses 'type' to indicate the field type and not the widget type. It is only the D7 migration which has this problem and hence I have only changed that YML file. I am wondering if we should make it consistent by setting 'type' to field_type and rename the existing 'type' to 'widget_type'. What do you think?