servo: Merge #10590 - Package tidy (from edunham:package-tidy); r=larsbergstrom

This fixes https://github.com/servo/servo/issues/861.

@askeing, I've copied your work from https://github.com/askeing/servo_tidy and attributed the commit to you. My commit in this PR is Git housekeeping to preserve `tidy`'s history. If you'd like to make additional changes, I've given you and @shinglyu push access to my fork of Servo. Apologies if this is already familiar, but the workflow for pushing to my branch is:

```
$ git remote add edunham git@github.com:edunham/servo.git
$ git checkout -b package-tidy
$ git pull edunham package-tidy
$ git push edunham package-tidy
```

Once this lands, I'll look at how to publish it to PyPI and automate that process.

Please don't merge this yet; we still need to discuss how the change should work around https://github.com/servo/servo/blob/master/python/servo/testing_commands.py#L33 , as I've yet to figure out how to get the egg to actually expose its tests.

Source-Repo: https://github.com/servo/servo
Source-Revision: bfe54539d290cb287e59e8ba106a54a3fab6201a

servo/python/mach_bootstrap.py

@@ -13,6 +13,7 @@ from pipes import quote
 
 SEARCH_PATHS = [
     os.path.join("python", "mach"),
+    os.path.join("python", "tidy"),
     os.path.join("tests", "wpt"),
     os.path.join("tests", "wpt", "harness"),
 ]