[bug] Directory structure requires `software.eessi.io`

[Neves-P]

With the current workflow, in order for modified easystack files to be picked up correctly, one needs to place them in a software.eessi.io subdirectory (see confirmation of this here: multixscale/dev.eessi.io-espresso#1 (comment); associated with EESSI/software-layer-scripts#8).

The necessary changes need to be made either in https://github.com/EESSI/software-layer-scripts/blob/main/EESSI-install-software.sh, or in the dev.eessi.io override variables.

[Neves-P]

@bedroge thought of a good way of solving this, which is somewhat of a refactor for all the overrides needed for dev.eessi.io. If we take a similar approach as he took here: https://github.com/EESSI/software-layer-scripts/blob/main/init/modules/EESSI/2023.06.lua#L32, then we can have most or all of the overrides neatly in one place.

We can check if $EESSI_DEV_PROJECT as a way to determine if the overrides should be applied/read.