Support different paths remapping for different scopes

I want to raise that I think it should be possible to apply different remapping to the same path for different scopes. It seems like remap-path-prefix was too generic in remapping across many different use cases, and now the scope argument is being introduced to limit the scope, but that doesn't deal with the situation where you want to remap the same path differently across scopes.

E.g. imagine I want to remap /home/cameron/code/project to "." for macro, but to "" for split-debuginfo. This would be necessary on macOS, where OSO stabs cannot use relative paths (see #132143).

You could imagine doing something like:

rustc ...
  --remap-path-scope=split-debuginfo \
  --remap-path-prefix=/home/cameron/code/project=. \
  --remap-path-scope=macro \
  --remap-path-prefix=/home/cameron/code/project=

But repeated usage of these flags are independently aggregated, rather than ordered, so that wouldn't work.

Not sure on the right design here, but I do think being able to express ^ is worthwhile.

Originally posted by @csmulhern in #111540

Start by reviewing rustc's existing --remap-path-scope and --remap-path-prefix handling, including the behavior described in issue #132143. Define how repeated flags should associate a path remapping with each scope, then verify that the example distinguishes macro and split-debuginfo remappings without changing unrelated scopes.