[6.0] Move CMS Filesystem package to compat plugin

[Hackwar]

Summary of Changes

This PR moves the CMS Filesystem classes to the compat plugin, allowing legacy third party code to still run when the plugin is active, but making the first step to finally remove it from the CMS.

I've added the Composer Dependency Changed label because the PR requires composer i to be run.

Testing Instructions

Copy the following line in a view of your choice to generate a call to the legacy classes:

var_dump(\Joomla\CMS\Filesystem\Folder::listFolderTree(JPATH_BASE, '.'));

Make sure the compat plugin is enabled AND you have to run composer i to generate an updated class loader file
Visit the view you edited in the browser to see an output of the call to the legacy class. Disable the plugin and see an error thrown instead.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[laoneo]

Please move the "classes" folder outside of the src folder. Like it is now you have the src folder managed by a classloader and then the "classes" folder managed by another classloader. I would also rename the "classes" folder to legacy or so.

[Hackwar]

Done

[sandewt]

Suggested change

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	<option value="0">JNO</option>
	>
	<option value="0">JNO</option>