Generate ETI files

[msimberg]

Nothing to see here yet. The implementation is as simple as possible right now but if this is where we want to go then it needs further cleanup before merging. It's not a clear win to split the ETI files into smaller files (some builds become faster, but there is overhead), but checking to see what the effect is in CI.

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

Generally speaking, rebuilds of single algorithms become faster since they can be faster, and full rebuilds become slower, since they had enough parallelism anyway. I'm leaving the PR as a draft, but I'm not going to do any further work on it for the time being as the benefit is not entirely clear.