Sources for extensions are still downloaded with `--module-only`

[ocaisa]

I was testing --module-only with the EESSI stack to build my module tree and my VM ran out of space. I thought it was the CVMFS cache (which was indeed partially to blame) but actually it turned out that the /tmp was full and inside I found a sources directory that was populated with lots of extension sources.

[boegel]

This happens because init_ext_instances which is triggered via _sanity_check_step_extensions calls fetch_extension_sources in the EasyBlock class (since #3655).

We need those extension instances for the sanity check, but we don't actually need the sources, so we should decouple creating extension instances and downloading sources (so breaking up fetch_extension_sources, which now does both).

[boegel]

Semi-large refactoring done in #3860 which fixes this...