(created using eb --new-pr)

When an extension has nosource: True set (see https://docs.easybuild.io/writing-easyconfig-files/?h=nosource#module_extensions) then self.src is an empty list

Hence we must not try to unpack the source.

Additionally we need a reasonable start_dir and optionally ext_dir. I choose to use the start_dir from the main EC. This also required a change in and related to _set_start_dir:

• A user defined start_dir (in extension options) is likely relative to the main EC start_dir
• If any start_dir is set (and exists) we want to change into that directory. This might be a no-op as we already are in the start_dir of the main EC if we don't unpack sources. And if we unpack sources we already do this directory change