The variant has on the left-hand side the topologically sorted vector
and the right-hand side is a pair showing the path and its parent that
represent a cycle in the graph making the sort impossible.
The goal is to implement #551 which needs to throw an error if the
topo-sort fails. However, the error-message is supposed to contain a
graph of store-paths and the API to generate this is inherently async.
Now, catching the exception and re-throwing another one is impossible
since `co_await` is forbidden in `catch`-blocks and adding another
topoSort variant that allows an async `makeError` also seems odd. Hence,
I decided to alter the data-structure in use a bit for this use-case.
One out of two uses of the function are affected after all.
Change-Id: I70a987f470437df8beb3b1cc203ff88701d0aa1b
Maximilian Bosch
f7871fcb57