Graph theory make head hurty. Closes#7367.
Basically, when we topologically sort the dependency graph, we already find its strongly-connected components (a really [neat algorithm][1]). This PR adds an algorithm that can dissect those into simple cycles, giving us more useful error reports.
test: `system_build_errors::dependency_cycle`
```
schedule has 1 before/after cycle(s):
cycle 1: system set 'A' must run before itself
system set 'A'
... which must run before system set 'B'
... which must run before system set 'A'
```
```
schedule has 1 before/after cycle(s):
cycle 1: system 'foo' must run before itself
system 'foo'
... which must run before system 'bar'
... which must run before system 'foo'
```
test: `system_build_errors::hierarchy_cycle`
```
schedule has 1 in_set cycle(s):
cycle 1: system set 'A' contains itself
system set 'A'
... which contains system set 'B'
... which contains system set 'A'
```
[1]: https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
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