Clarify/replace 'stage' terminology in Package Manager

Problem/Motivation

Follow-up to ✨ Add Alpha level Experimental Package Manager module Needs review .

Package manager currently uses the word 'stage' to mean several things.

1. A 'staging environment' as in composer stager. @phenaproxima mentioned changing this to sandbox in https://git.drupalcode.org/project/drupal/-/merge_requests/3608#note_376550

2. As a verb, i.e. to 'stage' the stage/sandbox environment.

3. As a synonym of 'phase' or 'step', or the object which manages the lifecycle of a staging directory.

For example:
StageEvent

"The stage event during which this exception is thrown."

"Stage operations"

I personally found this quite confusing when reviewing the code, it's hard to scan something and see what it's referring to at any one time.

Steps to reproduce

Proposed resolution

Not sure. I think when referring to a staging/sandbox directory, it would be good to consistently add 'directory' every time.

But if we can use actual different words for different things (sandbox, phase, step?) that would be better.

Remaining tasks

Opening this as a beta blocker because it is very likely to involve some class renames, even if the only consumers of the API are automatic updates and project browser - but that could get complicated with the contrib versions.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet