import: support importing resources into nested stacks

### Describe the feature

I would like to be able to import into the nested stack in the following structure:
```
Stack
NestedStack
ResourceToImport
```

### Use Case

I have nested stacks with resources that need to be imported

### Proposed Solution

I am not familiar with all the implementation details of `import` but some options that come to mind are:
- Treat nested stacks like root stacks in that you can select them for import (e.g. `cdk import MyStack/MyNestedStack`)
- When import is run on a root stack, it is recursively ran on all nested stacks under that stack

### Other Information

The implementation of this feature may have some interplay with the implementation of #20917 since both have to do with the selection and processing of stacks for `import`

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.29.1

### Environment details (OS name and version, etc.)

Windows 10 Enterprise Version 10.0.19044

The issue names no files or tests; start by tracing the CDK import entry point and how stacks are selected and processed, then compare the related concerns in #20917. Done means resources can be imported into a nested stack, either by selecting that stack directly or through recursive root-stack processing, with the behavior covered by tests.

Written by the indexing model from the issue text.