rustdoc: Add support for --remap-path-prefix
Adds `--remap-path-prefix` as an unstable option. This is implemented to mimic the behavior of `rustc`'s `--remap-path-prefix`.
This flag similarly takes in two paths, a prefix to replace and a replacement string.
This is useful for build tools (e.g. Buck) other than cargo that can run doc tests.
cc: `@dtolnay`
6d94a87275