fix: deep delete for views without navigation

[larslutz96]

Support deep delete when navigation is not included in the view.
Rewrite first incoming query in deep_delete() to a delete from database table where keys in select from view and call onDELETE again with this query.

[BobdenOs]

Looks good. Still very compact solution.

[larslutz96]

@BobdenOs
Johannes mentioned a scenario:

service S {
  entity RootP as projection on Root {
        key ID,
        children
    };

   entity ChildP as projection on Child where x = 1
}

DELETE.from(RootP(5)) deletes root with id 5 and all children. Johannes said:
I think that this is not what we want. The compiler redirects the composition of RootP to the filtered projection ChildP. Hence, I believe we must respect the modeled where clause and must not delete children which are excluded by the projection.

It also contradicts the behavior of deep update where we compare the data on projection level which respects the where clause.

[patricebender]

👍 before merging, would you please update the PR title and explain your approach in the PR description in more detail? Thanks!

[danjoa]

Had to revert this commit as it broke our internal pipelines du to the incompatible override of db.exists().
I'd also like to have a review of the implementation: I'm not in favour of reusing more internal @sap/cds functions