Require Python file handers to import helpers via root path (web-platform-tests#26328)

This changes handlers.py to no longer play with `sys.path` and `sys.modules`, and instead require python file handlers import helper functions with root-relative paths. `__init__.py` files are added as required.

Changing this did require changing imports in multiple tests; this can be reverted once we get to Py3-only (and pep 420) and can remove the root `__init__.py`.

Docs and tests are updated as appropriate.

See web-platform-tests/rfcs#68

Author: stephenmcgruer

__init__.py

@@ -2,8 +2,8 @@
 
 from wptserve.utils import isomorphic_decode, isomorphic_encode
 
-sys.path.insert(0, os.path.dirname(os.path.abspath(isomorphic_decode(__file__))))
-import util
+import importlib
+util = importlib.import_module("common.security-features.scope.util")
 
 def main(request, response):
   policyDeliveries = json.loads(request.GET.first(b"policyDeliveries", b"[]"))

common/__init__.py

@@ -1,8 +1,8 @@
 import os, sys, json
 
 from wptserve.utils import isomorphic_decode, isomorphic_encode
-sys.path.insert(0, os.path.dirname(os.path.abspath(isomorphic_decode(__file__))))
-import util
+import importlib
+util = importlib.import_module("common.security-features.scope.util")
 
 def main(request, response):
   policyDeliveries = json.loads(request.GET.first(b'policyDeliveries', b'[]'))

common/security-features/__init__.py

@@ -1,7 +1,7 @@
 import os, sys
 from wptserve.utils import isomorphic_decode
-sys.path.insert(0, os.path.dirname(os.path.abspath(isomorphic_decode(__file__))))
-import subresource
+import importlib
+subresource = importlib.import_module("common.security-features.subresource.subresource")
 
 def generate_payload(request, server_data):
     file = os.path.join(request.doc_root, u"webaudio", u"resources",

common/security-features/scope/__init__.py

@@ -1,7 +1,7 @@
 import os, sys
 from wptserve.utils import isomorphic_decode
-sys.path.insert(0, os.path.dirname(os.path.abspath(isomorphic_decode(__file__))))
-import subresource
+import importlib
+subresource = importlib.import_module("common.security-features.subresource.subresource")
 
 def generate_payload(server_data):
     return subresource.get_template(u"document.html.template") % server_data

common/security-features/scope/document.py

@@ -1,7 +1,7 @@
 import os, sys
 from wptserve.utils import isomorphic_decode
-sys.path.insert(0, os.path.dirname(os.path.abspath(isomorphic_decode(__file__))))
-import subresource
+import importlib
+subresource = importlib.import_module("common.security-features.subresource.subresource")
 
 def generate_payload(server_data):
     return u''

common/security-features/scope/worker.py

@@ -1,7 +1,7 @@
 import os, sys, base64
 from wptserve.utils import isomorphic_decode
-sys.path.insert(0, os.path.dirname(os.path.abspath(isomorphic_decode(__file__))))
-import subresource
+import importlib
+subresource = importlib.import_module("common.security-features.subresource.subresource")
 
 def generate_payload(request, server_data):
     data = (u'{"headers": %(headers)s}') % server_data