Make `resolve_full_path` use the appropriate separators, and return a
`String`.
As part of fixing the fallout from that type change, this also fixes a
bunch of places in the agent code that were using `std::path::Path`
operations on paths that could be non-local, by changing them to operate
instead on strings and use the project's `PathStyle`.
This clears the way a bit for making `full_path` also return a string
instead of a `PathBuf`, but I've left that for a follow-up.
Release Notes:
- N/A
d83d7d35cb