Only scaffold files when scaffold package is changed

Problem/Motivation

Currently, the Composer scaffolding plugin always re-scaffolds the project on install or update.

This means that any complications associated with a given scaffolding will repeat even if the package that installed the scaffold was not updated.

Proposed resolution

Only update the scaffolding when the scaffold package is changed.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet