Validate missing imports in CI (#615)

Author: Lukasa

docker/docker-compose.2004.main.yaml

@@ -10,7 +10,8 @@ services:
 
   test:
     image: async-http-client:20.04-main
-    environment: []
+    environment:
+      - IMPORT_CHECK_ARG=--explicit-target-dependency-import-check error
       #- SANITIZER_ARG=--sanitize=thread
 
   shell:

docker/docker-compose.yaml

@@ -28,7 +28,7 @@ services:
 
   test:
     <<: *common
-    command: /bin/bash -xcl "swift test --parallel -Xswiftc -warnings-as-errors --enable-test-discovery $${SANITIZER_ARG-}"
+    command: /bin/bash -xcl "swift test --parallel -Xswiftc -warnings-as-errors --enable-test-discovery $${SANITIZER_ARG-} $${IMPORT_CHECK_ARG-}"
 
   # util