Revert "Extensions: make loading of extensions independent of what packages are in the sysimage (#52841)

[KristofferC]

This reverts commit 08d229f4a7cb0c3ef5becddd1b3bc4f8f178b8e4.

There are some bugs now where extensions do not load when their package has been put into the sysimage. #52841 was made because it was common to get cycles otherwise but with https://github.com/JuliaLang/julia/pull/55589 that should be much less of a problem.

Subsumes https://github.com/JuliaLang/julia/pull/54750.