Have pub deps --json quiet when no terminal is attached (#4602)

Author: sigurdm

lib/src/command/deps.dart

@@ -99,7 +99,12 @@ class DepsCommand extends PubCommand {
       ];
       final toVisit = [...workspacePackageNames];
       final packagesJson = <dynamic>[];
-      final graph = await entrypoint.packageGraph;
+
+      // Avoid polluting stdout when outputting json.
+      final graph = await log.errorsOnlyUnlessTerminal(
+        () async => await entrypoint.packageGraph,
+      );
+
       while (toVisit.isNotEmpty) {
         final current = toVisit.removeLast();
         if (visited.contains(current)) continue;