Automate meta file check (#2133)

* Script to validate .meta files are set up correctly

* Add command to CI

* don't gitignore Gizmos, add .meta

* Move to utils directory

.circleci/config.yml

             black --check ml-agents-envs
             black --check gym-unity
 
+      - run:
+          name: Verify there are no hidden/missing metafiles.
+          # Renaming files or deleting files can leave metafiles behind that makes Unity very unhappy.
+          command: |
+            . venv/bin/activate
+            python utils/validate_meta_files.py
+
       - store_test_results:
           path: test-reports
 

.gitignore

 /UnitySDK/[Uu]nity[Pp]ackage[Mm]anager/
 /UnitySDK/Assets/AssetStoreTools*
 /UnitySDK/Assets/Plugins*
-/UnitySDK/Assets/Gizmos*
 /UnitySDK/Assets/Demonstrations*
 
 # Tensorflow Model Info

UnitySDK/Assets/Gizmos.meta

+fileFormatVersion: 2
+guid: e74b9e5c364014df1a24c696734cc461
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

utils/validate_meta_files.py

+import json
+import os
+
+
+def main():
+    asset_path = "UnitySDK/Assets"
+    meta_suffix = ".meta"
+    python_suffix = ".py"
+
+    num_matched = 0
+
+    unmatched = set()
+
+    for root, dirs, files in os.walk(asset_path):
+        dirs = set(dirs)
+        files = set(files)
+
+        combined = dirs | files
+        for f in combined:
+            if f.endswith(python_suffix):
+                # Probably this script; skip it
+                continue
+
+            # We expect each non-.meta file to have a .meta file, and each .meta file to have a non-.meta file
+            if f.endswith(meta_suffix):
+                expected = f.replace(meta_suffix, "")
+            else:
+                expected = f + meta_suffix
+
+            if expected not in combined:
+                unmatched.add(os.path.join(root, f))
+            else:
+                num_matched += 1
+
+    if unmatched:
+        raise Exception(f"Mismatch between expected files and their .meta files: {sorted(unmatched)}")
+
+    print(f"Found {num_matched} correctly matched files")
+
+
+if __name__ == "__main__":
+    main()