[FEATURE] Add possibility to use an extension as repository

Sometimes extensions depend on customized versions of other
extensions or on extensions that aren't released to the TER.

This feature allows an extension to deliver it's own dependencies
by having the unpacked extensions inside the folder
Initialisation/Extensions. The extensions are then copied to
typo3conf/ext when resolving the dependencies.

Resolves: #51835
Releases: 6.2
Change-Id: I66f6e5d5d40e11471f9766ac2ed9d0eb5386eb49
Reviewed-on: https://review.typo3.org/23672
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind