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

lib/src/command/deps.dart

diff --git a/lib/src/command/deps.dart b/lib/src/command/deps.dart
index c3cd18f..32c0a0b 100644
--- a/lib/src/command/deps.dart
+++ b/lib/src/command/deps.dart
@@ -99,7 +99,12 @@
       ];
       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;