[turbopack] Make internal functions private and use rcstr! macro (#81041)
## Refactor resolve module and improve RcStr usage
### What?
- Made several internal methods private that were unnecessarily public
- Removed unused methods like `ResolveResult.map()`
- Removed redundant fragment resolution code path
- Improved RcStr usage by using `rcstr!` macro instead of `.into()` conversions
- Fixed string handling in resolve functions to use RcStr consistently
### Why?
I was taking a close look at `resolve` to understand how it worked and stumbled across these improvements