cache: rename .dvc/cache/run to .dvc/cache/runs (#3732)

To be consistent with how git does it (e.g. objects) as well as with
future cache structure where we plan on adding `dirs`/`files`/`objects`
subdirs in the future.

dvc/stage/cache.py

@@ -37,7 +37,7 @@ def _get_stage_hash(stage):
 
 class StageCache:
     def __init__(self, cache_dir):
-        self.cache_dir = os.path.join(cache_dir, "run")
+        self.cache_dir = os.path.join(cache_dir, "runs")
 
     def _get_cache_dir(self, key):
         return os.path.join(self.cache_dir, key[:2], key)

tests/func/test_repro.py

@@ -1295,7 +1295,7 @@ class TestReproNoCommit(TestRepro):
             ["repro", self._get_stage_target(self.stage), "--no-commit"]
         )
         self.assertEqual(ret, 0)
-        self.assertEqual(os.listdir(self.dvc.cache.local.cache_dir), ["run"])
+        self.assertEqual(os.listdir(self.dvc.cache.local.cache_dir), ["runs"])
 
 
 class TestReproAlreadyCached(TestRepro):