dvc: switch from yaml to json for dir cache file

yaml is extremely slow without CLoader, which caused our checkout()
and similar functions to take way too long to process dir cache files.
These files are not really meant to be read by the users, so we can
sacrifice the readability if yaml for better performance of json module.

E.g. checkout for 30K files used to take 30sec and after this patch only
takes 6sec, which is still quite slow, but much more bearable.

Signed-off-by: Ruslan Kuprieiev <[email protected]>

dvc/output.py

@@ -1,6 +1,6 @@
 import os
 import stat
-import yaml
+import json
 from checksumdir import dirhash
 
 from dvc.system import System
@@ -249,7 +249,7 @@ class Output(Dependency):
 
         try:
             with open(path, 'r') as fd:
-                d = yaml.safe_load(fd)
+                d = json.load(fd)
         except Exception as exc:
             msg = u'Failed to load dir cache \'{}\''
             Logger.error(msg.format(relpath), exc)
@@ -349,7 +349,7 @@ class Output(Dependency):
             os.makedirs(dname)
 
         with open(self.cache, 'w+') as fd:
-            yaml.safe_dump(dir_info, fd, default_flow_style=False)
+            json.dump(dir_info, fd)
 
     def save(self):
         super(Output, self).save()