shellFor doesn't filter based on transitive dependencies

[TeofilC]

The packages selected by shellFor.packages are not meant to be built by shellFor.

This breaks when, say, you have three packages A, B, C, such that A depends on B, and B depends on C.
shellFor {packages = ps: [ps.A ps.B];} would correctly not build either A or B. But shellFor {packages = ps: [ps.A ps.C];} would build C, since A requires B, so B is built. B in turn requires C and so C is also built.

The issue is that shellFor is only filtering out direct dependencies. We also want to filter out packages that transitively depend on things in packages. So, shellFor {packages = ps: [ps.A ps.C];} shouldn't build A, B, or C.

[TeofilC]

I have an idea of how to fix this in a vaguely efficient way and will give implementing it a go over this weekend

[michaelpj]

Remarkably, I have also realised that this affects me recently. In my case it's due to extremely questionable cross-repository dependencies: my A and C are in repo 1 and B is in repo 2, all linked up with source-repository-packages 🙈

This is a bit complex, though, since we then need to put other dependencies of B in the shell, so that it can definitely be built inside the shell. Complicated.