servo: Merge #12447 - All our Cargo.toml files should contain an MPL-2.0 license field (from asajeffrey:license-mpl-2.0); r=frewsxcv

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Added a `license = "MPL-2.0"` field to all our `Cargo.toml` files, and added a check to `test-tidy` that the license is present.

---
<!-- 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` does not report any errors
- [X] These changes fix #12434
- [X] There are tests for these changes

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Source-Repo: https://github.com/servo/servo
Source-Revision: 650d1d1185eecdc4c8b605bb0290ece7de839015

servo/python/tidy/servo_tidy/tidy.py

@@ -17,7 +17,7 @@ import site
 import StringIO
 import subprocess
 import sys
-from licenseck import licenses
+from licenseck import licenses, licenses_toml
 
 # License and header checks
 EMACS_HEADER = "/* -*- Mode:"
@@ -295,9 +295,14 @@ duplicate versions for package "{package}"
 def check_toml(file_name, lines):
     if not file_name.endswith(".toml"):
         raise StopIteration
+    ok_licensed = False
     for idx, line in enumerate(lines):
         if line.find("*") != -1:
             yield (idx + 1, "found asterisk instead of minimum version number")
+        for license in licenses_toml:
+            ok_licensed |= (license in line)
+    if not ok_licensed:
+        yield (0, ".toml file should contain a valid license.")
 
 
 def check_rust(file_name, lines):