McuSupportPlugin: unify qtForMcusPackage with McuPackages

Follow-up to https://bugreports.qt.io/browse/QTCREATORBUG-26897

An instance of McuTarget contains a list of McuPackages, and an extra package for the SDK itself, called in some parts of the code qtForMCUsPackage.

The main reason to keep it separated was because it contained the AutomaticKitCreation flag (which is now in McuSupportOptions), and because the path it points to is addressed in some parts of the code.

However, storing it separately from the targets and having to pass it along with every function that manages targets is cumbersome and error-prone. Targets should store a reference to this special instance, in order to simplify processing them.