[TASK] Move toplevel index.php to frontend extension

This moves the toplevel "index.php" to the frontend extension
as "frontend.php". The toplevel "index.php" becomes a stub which
includes the "frontend.php".

This setup works in the following cases:

1) Package "typo3/cms" is the root package
2) The "index.php" is symlinked into the web-dir
3) The "index.php" is copied into the web-dir

In all of these cases "typo3" is a toplevel directory.

Resolves: #73027
Releases: master
Change-Id: I1c0653bcc4691105eb2ccfbea2bcc873ce6189da
Reviewed-on: https://review.typo3.org/46390


Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>