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/importer/__init__.py

@@ -0,0 +1,57 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+import os
+import sys
+
+from .tree import GitTree, GeckoCommit
+
+from wptrunner.update import setup_logging, WPTUpdate  # noqa: F401
+from wptrunner.update.base import Step, StepRunner, exit_unclean  # noqa: F401
+from wptrunner.update.update import LoadConfig, SyncFromUpstream, UpdateMetadata  # noqa: F401
+from wptrunner import wptcommandline  # noqa: F401
+
+
+class LoadTrees(Step):
+    """Load gecko tree and sync tree containing web-platform-tests"""
+
+    provides = ["local_tree", "sync_tree"]
+
+    def create(self, state):
+        if os.path.exists(state.sync["path"]):
+            sync_tree = GitTree(root=state.sync["path"])
+        else:
+            sync_tree = None
+
+        assert GitTree.is_type()
+        state.update({"local_tree": GitTree(commit_cls=GeckoCommit),
+                      "sync_tree": sync_tree})
+
+
+class UpdateRunner(StepRunner):
+    """Overall runner for updating web-platform-tests in Gecko."""
+    steps = [LoadConfig,
+             LoadTrees,
+             SyncFromUpstream,
+             UpdateMetadata]
+
+
+def run_update(**kwargs):
+    logger = setup_logging(kwargs, {"mach": sys.stdout})
+    updater = WPTUpdate(logger, runner_cls=UpdateRunner, **kwargs)
+    return updater.run() != exit_unclean
+
+
+def create_parser():
+    parser = wptcommandline.create_parser_update()
+    parser.add_argument("--layout-2020", "--with-layout-2020", default=False, action="store_true",
+                        help="Use expected results for the 2020 layout engine")
+    return parser
+
+
+def check_args(kwargs):
+    wptcommandline.set_from_config(kwargs)
+    if hasattr(wptcommandline, 'check_paths'):
+        wptcommandline.check_paths(kwargs)
+    return kwargs