[TASK] Use relative Git submodule URLs

By using relative submodule URLs, we can make use of Gerrit's submodule
subscription feature.

This automatically updates the Core repository, if a change is merged in
a submodule (e.g. extbase).

For end users, these are the consequences:

* It is easier, to have a TYPO3 mirror on an own server, as the
  submodules also point to the same server, instead of to typo3.org
* Users, which have the git port blocked, can clone the Core through
  ssh/https and submodules will be automatically transferred through the
  same protocol
* No change is required for users with a checkout. If they issue a git
  submodule sync, their .git/config will be updated (and the url for the
  submodules will be calculated based on the URL of the default remote +
  the relative URLs of the submodules.

Change-Id: I908a1bb07b254bf749b5476dd957dbb4a81337e2
Resolves: #46213
Releases: 6.1, 6.0, 4.7, 4.5