graph: use find_cycle instead of simple_cycle (#2668)

We are incorrectly using `simple_cycle` here, `find_cycle` is much more
suitable for this application and much faster too. For example:

```
$ cat test_networkx.py
import networkx as nx
from dvc.repo.graph import check_acyclic

G = nx.scale_free_graph(1200)

list(nx.find_cycle(G, orientation='ignore'))

list(nx.simple_cycles(G))

$ python -mcProfile -scumtime test_networkx.py &> log.log
```

shows ~100s for `simple_cycles` and ~0s for `find_cycle`.

Fixes #2666

dvc/repo/graph.py

@@ -5,10 +5,17 @@ def check_acyclic(graph):
     import networkx as nx
     from dvc.exceptions import CyclicGraphError
 
-    cycle = next(nx.simple_cycles(graph), None)
+    try:
+        edges = nx.find_cycle(graph, orientation="original")
+    except nx.NetworkXNoCycle:
+        return
 
-    if cycle:
-        raise CyclicGraphError(cycle)
+    stages = set()
+    for from_node, to_node, _ in edges:
+        stages.add(from_node)
+        stages.add(to_node)
+
+    raise CyclicGraphError(list(stages))
 
 
 def get_pipeline(pipelines, node):