Runfiles API: obtaining the symlink path within runfiles_dir

### Description of the problem / feature request:

This is a feature request for an API addition to the runfiles library. I would like to have a way to return a runfiles path that lives inside the `$RUNFILES_DIR` (which will be symlink to its actual home), rather than having the runfiles library return the symlink's destination.

Chasing the symlink inside the `Runfiles` library loses information. Details below.

### Feature requests: what underlying problem are you trying to solve with this feature?

Consider this demo program: https://github.com/jwnimmer-tri/repro/tree/bazel-runfiles-paths/demo

The `program` uses two runfiles: the `mesh.obj` which is part of the source tree, and the `mesh.mtl` which is the output by a genrule. As is conventional for [obj files](https://en.wikipedia.org/wiki/Wavefront_.obj_file) the text of the obj mentions a material library filename by saying `mtllib mesh.mtl`, and refers to a file in the same directory. For any software to load that file, the two files (obj and mtl) must be in the same directory.

However, if I run the program I see this output (bazel 8.0.0, rules_cc 0.0.17):

```console
$ bazel run //demo:program
The goal is for all of these paths to be in the same directory:

mesh_path: /home/jwnimmer/jwnimmer-tri/repro/demo/mesh.obj
mtl_path: /mnt/nobackup/cache/bazel/_bazel_jwnimmer/d99b0c65cbdeed7837d9eb064003636b/execroot/_main/bazel-out/k8-fastbuild/bin/demo/mesh.mtl

RUNFILES_DIR=/mnt/nobackup/cache/bazel/_bazel_jwnimmer/d99b0c65cbdeed7837d9eb064003636b/execroot/_main/bazel-out/k8-fastbuild/bin/demo/program.runfiles and contains these files:
MANIFEST
_main
_main/demo
_main/demo/mesh.mtl
_main/demo/mesh.obj
_main/demo/program
_repo_mapping
```

When the program calls `Rlocation`, the result is a path to the actual file -- for the obj we get the source tree and for the generated file we get the output path. That's fine in many cases, but when we have groups of inter-related related files and some of them are source files others are build outputs, we often need to have them in a single logical directory even if stored elsewhere. This is a requirement of many file formats that weave together multiple files into a single entity, to have their sub-asset files in the same directory as the main file.

It turns out we already have a reasonable directory with logical filenames no matter where they physically came from -- the files under RUNFILES_DIR are laid out logically, as seen in the RUNFILES_DIR walk above.

I would like to be able to call Rlocation and get paths like this:
- `/mnt/nobackup/cache/bazel/_bazel_jwnimmer/d99b0c65cbdeed7837d9eb064003636b/execroot/_main/bazel-out/k8-fastbuild/bin/demo/program.runfiles/_main/demo/mesh.obj`
- `/mnt/nobackup/cache/bazel/_bazel_jwnimmer/d99b0c65cbdeed7837d9eb064003636b/execroot/_main/bazel-out/k8-fastbuild/bin/demo/program.runfiles/_main/demo/mesh.mtl`

### Proposed solution

If the `class Runfiles` object had some kind of flag or option to opt-in to returning `RUNFILES_DIR` relative paths, instead of the manifest paths, that would solve the problem. It could either be a constructor argument, or an option argument to `Rlocation`.

### Work-arounds

Prior to bzlmod, I could scrape the `EnvVars` for `RUNFILES_DIR` and then tack on the workspace name and resource path afterward, to find the shape of path I need. Now with bzlmod, we have the `_repo_mapping` rewriting happening, which is not realistically possible to re-implement myself. My current work-around is to `#define private public` and then `clear()` the `runfiles_map_` private member, in which case the Runfiles object always returns relative paths to the runfiles dir.

### What operating system are you running Bazel on?

Ubuntu 22.04

### What's the output of `bazel info release`?

release 8.0.0

### What version of rules_cc do you use? Can you paste the workspace rule used to fetch rules_cc? What other relevant dependencies does your project have?

See example link above for the `MODULE.bazel` of the reproducer. Only `rules_cc == 0.0.17`.

### What Bazel options do you use to trigger the issue? What C++ toolchain do you use?

See example link above for reproducer. Local toolchain.

### Have you found anything relevant by searching the web?

The https://groups.google.com/g/bazel-discuss/c/DsVivJhU7Bw discussion is loosely relevant.

### Any other information, logs, or outputs that you want to share?

N/A