Feature request: Dependents of multiple packages

Sometimes it may be useful to have a list of all packages that have a dependency on two (or more packages).

For example,
Lets say i have the package `me/root-package`, which depends on `other/depend ^2.0`. Now i wish to upgrade it to 3.0, but i am unsure if i still want to support 2.0 or drop support for that completely.

Searching what packages are depending on both `me/root-package` , and `other/depend ` can give me an indication of how many users may be affected, and what versions those users are requiring of that package.

The issue names no files, tests, or entry points. Start by locating the existing dependent-package search and tracing how dependency constraints are queried; then determine how multiple packages and their required versions should be represented. Done means users can find packages depending on all requested packages and inspect the relevant version constraints.

Written by the indexing model from the issue text.