chore: Using local sflight clone instead of git dependency

[danjoa]

Simplifies our setup, and speeds up pipelines

[BobdenOs]

My main concern is that copying sflight and bookshop into the repository is that they will go out of sync and not just that people have already modified the bookshop model. Which causes trouble where sometimes the main state of the bookshop does not work while the cds-dbs state of the bookshop is passing without any issues.

If using sub-modules would provide the same performance benefit as it would be git cloned from github.com anyway. That would be my preference as it doesn't allow people to modify the copy and will make it straight forward to keep the state up to date.

[danjoa]

Agreed, but we anyways have to find better ways to deal with that ... i.e. we need some umbrella-kind-of setup like cap/dev available externally in public GitHub. That also is the preliminary conclusion from respective considerations with Daniel around Incidents Mgmt and the Calesi repos (which all live in public Github).

This PR is a near-term measure to fix problems we have with always having a duplicate install of SFlight in cap/dev today, as well as the problem that Steffen encountered recently when he had to revert a change he thought to do locally in SFlight which broke tests in cds-dbs. → I'd propose to merge that for now and follow up on the bigger thing near term.