Allow dynamic imports in sandbox example

I'd like to execute code using the sandbox example, but am running into an issue. I want to allow exec'd code to import packages, but if I don't import packages in the host script, the guest code won't run.

```python
# Guest code
import json

print(json.dumps({"message": "Hello"}))
```

```python
# Works
import sys
import json

from command import exports

class Run(exports.Run):
def run(self):
with open(sys.argv[1]) as f:
code = f.read()
exec(code)
```

```python
# Fails
import sys

from command import exports

class Run(exports.Run):
def run(self):
with open(sys.argv[1]) as f:
code = f.read()
exec(code)
```

从 sandbox 示例的 Run.run 入口点开始，尤其关注它读取 guest 代码并调用 exec 的位置。分别在 host 端导入和不导入 json 的情况下运行所示的 guest 程序，然后跟踪 import 行为。当 guest 代码可以动态导入 json，而不要求 host 脚本先导入它时，即表示完成。

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