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)
```

Start at the sandbox example's Run.run entry point, especially where it reads the guest code and calls exec. Run the shown guest program with and without the host-side json import, then trace the import behavior. Done means guest code can dynamically import json without requiring the host script to import it first.

索引モデルが issue の本文から書いたものです。