Delay import to allow proper debian packaging of the client code

The swh.graph.client and swh.graph.naive_client must be packaged separately to allow the
vault to use the optional graph dependency. We need the tests to run in the packaging
though. The current conftest.py mixes both server and client code though. Delaying
imports this way will allow the packaging to run accordingly [1].

[1]
```
pytest -k "not remote" swh/
```

Related to T3518

swh/graph/tests/conftest.py

@@ -10,11 +10,8 @@ from pathlib import Path
 from aiohttp.test_utils import TestClient, TestServer, loop_context
 import pytest
 
-from swh.graph.backend import Backend
 from swh.graph.client import RemoteGraphClient
-from swh.graph.graph import load as graph_load
 from swh.graph.naive_client import NaiveClient
-from swh.graph.server.app import make_app
 
 SWH_GRAPH_TESTS_ROOT = Path(__file__).parents[0]
 TEST_GRAPH_PATH = SWH_GRAPH_TESTS_ROOT / "dataset/output/example"
@@ -26,6 +23,10 @@ class GraphServerProcess(multiprocessing.Process):
         super().__init__(*args, **kwargs)
 
     def run(self):
+        # Lazy import to allow debian packaging
+        from swh.graph.backend import Backend
+        from swh.graph.server.app import make_app
+
         try:
             backend = Backend(graph_path=str(TEST_GRAPH_PATH))
             with loop_context() as loop:
@@ -60,5 +61,8 @@ def graph_client(request):
 
 @pytest.fixture(scope="module")
 def graph():
+    # Lazy import to allow debian packaging
+    from swh.graph.graph import load as graph_load
+
     with graph_load(str(TEST_GRAPH_PATH)) as g:
         yield g