Display machine name should match render array element

Problem/Motivation

Currently the display machine name in commerce_pvt_entity_extra_field_info() is commerce_pvt_field while the the render array element in commerce_pvt_commerce_product_view() is commerce_pvt.

This mismatch means the position of the field can't be set in the UI.

Steps to reproduce

1. Install Commerce, Commerce PVT etc. and add a product with variations
2. Install Field Layout (we're also using Field Group)
3. In "Manage display" for the product, select a layout, (optionally add a field group) and position the Commerce PVT field in a column/region

The Commerce PVT field always appears outside the selected layout.

Proposed resolution

Set the display machine name and component name to the same as the render array element.

Remaining tasks

User interface changes

API changes

Data model changes