Manual toolchain propagation in `query` mode

Bazel-diff in query mode doesn't see toolchain changes, but this is a Bazel limitation so we can't do this automatically.

In our codebase, we have a compiler that feeds into a toolchain. In order to detect changes to this, we have this workaround that we apply in a post-processing step (among other things):

    # Hack: We need to use `bazel cquery` to detect toolchain changes
    # (see https://github.com/Tinder/bazel-diff/issues/101),
    # but if we used cquery we would have to run this on _every_ platform.
    # The only case this notably affects for us is that if Mojo changes,
    # all Mojo targets should change. So special case this.
    if grep --quiet //KGEN/tools/mojo "$output_file.raw"; then
        echo "- Mojo changed, manually adding Mojo targets"
        ./bazelw query 'rdeps(//..., kind("mojo_*", //...))' >> "$output_file.mojo_targets"
        cat "$output_file.mojo_targets" >> "$output_file.raw"
    fi

And then we have to re-do a lot of processing (like excluding manual targets, etc). It would be interesting if we could support this natively. Generally, this would be a way of adding edges that query can't find.

Start by tracing query-mode result processing and the post-processing path represented by $output_file, including the bazelw query workaround for Mojo targets. Define how manually supplied edges should enter the query results and how existing exclusion and processing steps should handle them; done means toolchain-related changes can be represented natively without this workaround.