Prepare for DocC (#1467)

# Motivation
We wanna use DocC for publishing docs.

# Modification
Setup our `Package.swift`  manifests to enable proper doc generation.

# Result
We can now use DocC to generate docs.

scripts/license-check.sh

@@ -110,6 +110,11 @@ check_copyright_headers() {
         drop_first=1
         expected_lines=15
         ;;
+      */Package@swift-*.*.swift)
+        expected_sha="$SWIFT_SHA"
+        drop_first=1
+        expected_lines=15
+        ;;
       *)
         expected_sha="$SWIFT_SHA"
         drop_first=0