Cache key doesn't change despite some changes in the dependency graph

Let's consider this example. Element A.bst build-depends on two unrelated elements B.bst and C.bst.

Now if I add a runtime-depends on C.bst to B.bst, the cache keys don't change. B's cache key doesn't take into account runtime dependencies, and A's cache key only takes into account the list of build dependencies, which hasn't changed (it's still B and C, plus whatever runtime dependencies they had).

The issue here is that A does have access to the dependency graph, not only the list of dependencies. So `self.dependencies(recurse=False)` would return B and C, and calling `dependencies()` on B will reveal the new dependency.

Most plugins don't go looking into the dependency graph, but for those that do this leads to the same cache key (and the element not getting rebuilt) even though the result would be different.

Start by tracing the cache-key calculation for an element and the dependency APIs described in the issue, comparing direct dependencies with the recursive graph. Reproduce the A/B/C example, then verify that adding B's runtime dependency on C changes A's cache key and causes affected elements to rebuild.

Written by the indexing model from the issue text.