Incorrect handling of runtime dependencies

[maralorn]

Currently we only determine dependencies by looking into derivations:

There we have inputDerivations and inputSrcs.

But there is actually another kind of dependency: storePaths can reference other store paths without their derivation directly depending on that derivation as an input.

[maralorn]

We can query this information with nix path-info --json, I don’t know, though, whether that is available before a download has started.

[maralorn]

Also: Apparently we are not loading all builds which are "waiting for lock" which can create gaps in the build tree.