Validate missing imports in CI (#26)

Author: Lukasa

docker/docker-compose.2004.main.yaml

@@ -14,6 +14,8 @@ services:
 
   test:
     image: swift-http-structured-headers:20.04-main
+    environment:
+      - IMPORT_CHECK_ARG=--explicit-target-dependency-import-check error
 
   shell:
     image: swift-http-structured-headers:20.04-main

docker/docker-compose.yaml

@@ -32,7 +32,7 @@ services:
 
   test:
     <<: *common
-    command: /bin/bash -xcl "swift test --enable-test-discovery -Xswiftc -warnings-as-errors $${SANITIZER_ARG-}"
+    command: /bin/bash -xcl "swift test --enable-test-discovery -Xswiftc -warnings-as-errors $${SANITIZER_ARG-} $${IMPORT_CHECK_ARG-}"
 
 
   # util