Breaking behavior change in the rules_rust runfiles library

Hello,

In v0.59.1 I could do the following things using the runfiles library (that I depended on):

- `runfiles::Runfiles::rlocation("path/to/dir")` would return the path to the directory
- `runfiles::Runfiles::rlocation("")` would return the path to the root runfiles directory

However, in v0.61.0, neither of the above behavior work. Worse, Bazel runtime behavior does not match test behavior (ie, the same code works with `bazel test` but will fail in `bazel run`.

I'm not really certain what the root cause of this behavior change is, but it seems the additional manifest based stuff is probably related..

If the above behaviors are not supported, is there an alternative?

I'll continue to dig to isolate the changes that broke the above behaviors.