servo: Merge #11210 - Start on Mach package (from edunham:mach-package); r=larsbergstrom

Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data:
- [ ] `./mach build -d` does not report any errors
  - these changes don't touch anything that mach build touches>
- [ ] `./mach test-tidy --faster` does not report any errors
  - Tidy errors on some dependencies that I think we'll need for real `package` but aren't using for android
- [X] These changes address #9918 (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because I don't think Mach has tests yet?

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: bc52617d3351d64cfd1479c5088b591370529f59

servo/python/mach_bootstrap.py

@@ -23,6 +23,7 @@ MACH_MODULES = [
     os.path.join('python', 'servo', 'build_commands.py'),
     os.path.join('python', 'servo', 'testing_commands.py'),
     os.path.join('python', 'servo', 'post_build_commands.py'),
+    os.path.join('python', 'servo', 'package_commands.py'),
     os.path.join('python', 'servo', 'devenv_commands.py'),
 ]
 
@@ -58,6 +59,11 @@ CATEGORIES = {
         'long': 'Interact with specific parts of the build system.',
         'priority': 20,
     },
+    'package': {
+        'short': 'Package',
+        'long': 'Create objects to distribute',
+        'priority': 15,
+    },
     'misc': {
         'short': 'Potpourri',
         'long': 'Potent potables and assorted snacks.',