servo: Merge #11243 - Move constellation into its own crate (from Ms2ger:constellation); r=nox

Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data:
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because refactoring

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

Source-Repo: https://github.com/servo/servo
Source-Revision: 872565bb4bd2883c6fe926da74e3b3e2b74e6d34

servo/python/tidy/servo_tidy/tidy.py

@@ -585,6 +585,9 @@ def collect_errors_for_files(files_to_check, checking_functions, line_checking_f
     if print_text:
         print '\rChecking files for tidiness...'
     for filename in files_to_check:
+        if not os.path.exists(filename):
+            continue
+
         with open(filename, "r") as f:
             contents = f.read()
             for check in checking_functions: