Allow `Rlocation()` also for existing folders

### What version of rules_go are you using?

0.50.1

### What version of gazelle are you using?

0.39.1

### What version of Bazel are you using?

7.0.2

### Does this issue reproduce with the latest releases of all the above?

### What operating system and processor architecture are you using?

Linux

### Any other potentially useful information about your toolchain?

### What did you do?

```go
r, err := runfiles.New()
if err != nil {
return "", err
}
repoBasePath, err := r.Rlocation("my_repo")
repoBasePath, err = r.Rlocation("my_repo/my_pkg")
```

### What did you expect to see?

The previous snippet should succeed without error and return the runfiles path for `my_repo` or `my_repo/my_pkg`.

### What did you see instead?

The call to `Rlocation()` returnes an error for runfiles paths like directories which are not files. This works in the runfiles implementation of other programming languages like C++:
```cpp
repoBasePath = Runfiles::Create("")->Rlocation("my_repo");
repoBasePath = Runfiles::Create("")->Rlocation("my_repo/my_pkg");
```

Start at the Go runfiles implementation of Rlocation() and run the reported snippet with existing repository and package directories. Compare its behavior with the C++ example in the issue; done means Rlocation("my_repo") and Rlocation("my_repo/my_pkg") return their runfiles paths without errors.

Written by the indexing model from the issue text.