Does your feature request relate to a problem?

We removed all our ext_emconf.php files when updating from TYPO3 10 LTS to 12 LTS.
This leads to issues like:

UnexpectedValueException: The package "e2_ces" depends on "e2/e2-core" which is not present in the system.

Describe the solution you would like

Provide native support for composer only projects with multiple TYPO3 extensions that declare their dependencies on each other (for loading order).

Optionally, describe alternatives you considered

One could keep ext_emconf.php.