[4.0] PSR4 namespace

[brianteeman]

This will hopefully resolve the remaining appveyor issues

PR for #27014

I assume that if the appveyor tests complete without any deprecated notices then this is correct

cc @wilsonge

[brianteeman]

@wilsonge all the appveyor deprecated notices have gone - no idea about anything else in thi8s PR so its up to you

[SharkyKZ]

Renaming files that exist in 3.x also won't work. See discussion #23725.

[wilsonge]

Case sensitive renaming definitely doesn't work 'as is' because the upgrades from 3.x will fail. @HLeithner was going to investigate a way of being able to do this which would unblock this PR

[brianteeman]

@HLeithner anything ?

[HLeithner]

Not ready yet...

[brianteeman]

@HLeithner just checking in to see if there has been any progress

[zero-24]

When there is a solution please also backport it to 3.10 / staging
https://github.com/joomla/joomla-cms/blob/staging/libraries/src/Filesystem/Support/Stringcontroller.php

[brianteeman]

No point in me fixing the conflicts until @HLeithner delivers his code

[jwaisner]

@HLeithner checking to see if the required code has been completed for this to be able to move forward.

[wilsonge]

Closing in favour of #31804 which should rename the files correctly