Modpacks can't be interdependent

Nobody has claimed this yet.

ContentDB considers packages as a whole. It collects all the dependencies of the mods in the modpack into a single list and this is used for dependency resolution and game support

Say you have the following two packages:

```
Alpha package with mods alpha1 and alpha2
Beta package with mods beta1

alpha1 depends on beta1
beta1 depends on alpha2
```

Minetest will successfully load these mods in the order `alpha2, beta1, alpha1`. However, ContentDB will see this as a dependency cycle between the two packages, as `Alpha` depends on `Beta` and `Beta` depends on `Alpha`.

For an example of this in the wild, see joe7575/techage_modpack and rheo/moreblocks.

Start by tracing ContentDB's dependency-resolution and package dependency aggregation entry points; no specific files or tests are named in the issue. Reproduce the Alpha/Beta example and compare package-level cycle detection with Minetest's mod load order. Done means valid interdependent modpacks are accepted without a false dependency cycle.

Written by the indexing model from the issue text.