Fix bug with `imports` field from a nested directory (#5366)
### Description
When importing a `#foo` subpath import, the resolved file is relative
the the `package.json` containing the `imports` field, not the file with
the import statement. This fixes subpath imports to use the proper
context path during that resolve.
### Testing Instructions
```
RUST_BACKTRACE=1 cargo nextest run -p turbopack-tests -E 'test(snapshot__imports__subpath_imports_nested)' --no-capture
```
Fixes WEB-1216
