Support multiple sources for exts_list

[Flamefire]

There are some applications which need to be installed as an extension of e.g. PythonBundle. For example TensorFlow and seemingly now PyTorch.

Especially those would/need to download external stuff at configure/build time which can be avoided by using multiple sources. This works well, when a simple EasyBlock is used, e.g. PythonPackage, and the sources is top-level. But for exts_list multiple sources are currently impossible.

In order to solve this support for multiple sources should be added

[VRehnberg]

I second this. I would like to see this to simplify the installation of a Quantum ESPRESSO module.

[boegel]

Another use case is OpenFold, where an additional file needs to be downloaded and copied into the installation directory, see easybuilders/easybuild-easyconfigs#17242