[5.x] Export starter kit package folder when it exists

[jesseleite]

The Problem

When making a kit composer updatable (by setting updatable: true introduced by #11064), you're likely going to want to provide your own package service provider class, PSR-4 namespace, vendor views, etc. that are separate from export_paths (which get installed into the users app, rather than remaining in vendor). Right now there is no way to automatically export those vendor files to your starter kit repo.

The Solution

In this PR, we allow you to create your own package folder in your dev application for all your package's vendor files. When running php please starter-kit:export, if we detect that a package folder exists, we'll export all of those files to your starter kit package's root path, and scope all export_paths to an export folder to keep things clean and free from filename conflict with installable export_paths.

For example, if this is your dev app...

├─ app
│  └─ file.php
├─ config
├─ resources
├─ etc
│ 
├─ package
│  ├─ src
│  │  └─ Tags
│  │     └─ Foo.php
│  ├─ resources
│  │  ├─ css
│  │  ├─ js
│  │  └─ views
│  │     ├─ default.antlers.html
│  │     └─ layout.antlers.html
│  └─ composer.json
│ 
├─ composer.json
└─ starter-kit.yaml

Then this is what the final export will look like...

├─ export
│  └─ app
│     └─ file.php
├─ src
│  └─ Tags
│     └─ Foo.php
├─ resources
│  ├─ css
│  ├─ js
│  └─ views
│     ├─ default.antlers.html
│     └─ layout.antlers.html
├─ composer.json
└─ starter-kit.yaml

This lets you manage your package's composer.json, etc. from the package folder, and the exporter and installer will intelligently put Everything in Its Right Place, just like Radiohead.

Todo

• Handle these changes in the exporter
• Handle these changes in the installer
• Test coverage
• Docs PR