Package manager's Stage classes should always fetch a fresh tempstore

Problem/Motivation

\Drupal\package_manager\StageBase::__construct fetches the temp store from the factory in the constructor.
This locks the temp store to the user at that time.
The current user can change in the life of the request.

Steps to reproduce

Proposed resolution

Remove the factory call from the constructor and put it in a method.
Remove the tempStore property
Change all calls to $this->tempStore to fetch a fresh store from the factory. Multiple calls in one method should be avoided - a local variable can be used.
Fix tests that do a container rebuild after changing users - this shouldn't be needed anymore

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet