Reorganize Servo's WPT Python scripts

This change moves all of Servo's WPT Python support scripts into one
directory as they were previously scattered throughout the directory
structure. This should allow more code reuse and make it easier to
understand how everything fits together.

The changes:

- `tests/wpt/update` → `python/wpt/importer`
- `etc/ci/upstream-wpt-changes/wptupstreamer` → `python/wpt/exporter`
- `etc/ci/upstream-wpt-changes/test.py` → `python/wpt/test.py`
- `etc/ci/upstream-wpt-changes/tests` → `python/wpt/tests`
- `tests/wpt/servowpt.py` →
    - `python/wpt/update.py`
    - `python/wpt/run.py`
- `tests/wpt/manifestupdate.py` → `python/wpt/manifestupdate.py`

This change also removes
 - The ability to run the `update-wpt` and `test-wpt` commands without
   using `mach`. These didn't work very well, because it was difficult
   to get all of the wptrunner and mach dependencies installed outside
   of the Python virtualenv. It's simpler if they are always run through
   `mach`.
- The old WPT change upstreaming script that was no longer used.

python/wpt/tests/add-non-utf8-file.diff

@@ -0,0 +1,7 @@
+diff --git a/tests/wpt/web-platform-tests/non-utf8-file.txt b/tests/wpt/web-platform-tests/non-utf8-file.txt
+new file mode 100644
+index 0000000..5a992e0
+--- /dev/null
++++ b/tests/wpt/web-platform-tests/non-utf8-file.txt
+@@ -0,0 +1 @@
++foo bér ÿ "