[Backtracing] Extra SPI for offline symbolicator (#88481)
- **Explanation**:
Add extra SPI to be able to override find methods (for offline
symbolication fetch).
- **Scope**:
No functional change to existing SPI, adds new SPI (and two methods
changed from `public` to `open`).
- **Issues**:
rdar://136978398
- **Original PRs**:
n/a
- **Risk**:
none
- **Testing**:
tested in a local build on Darwin and Linux
- **Reviewers**:
@al45tair
Co-authored-by: Carl Peto <carlpeto@Carls-MacBook-Pro.local>
