Runfiles `Rlocation()` call without `source_repo` param fails in custom python package

# 🐞 bug report

### Affected Rule

`python.runfiles` `Rlocation()`

### Is this a regression?

No

### Description

When we call `Rlocation()` inside of a custom python package without passing in the `source_repo=` parameter and import our package for use elsewhere, if `sys.path` includes a path to our package's code that is outside the runfiles directory, then the `Rlocation()` call will fail.

Specifically, this is caused by the code [here](https://github.com/bazel-contrib/rules_python/blob/afab5ed2a6bc587676d9ab4022bccf977cc8566a/python/runfiles/runfiles.py#L433-L437), which attempts to look up runfiles using the repository mapping of the caller of the `Rlocation()` method. If `sys.path` has found and run the copy of our python package that resides outside of the runfiles directory, then the `Rlocation()` call is legitimately coming from outside the runfiles directory, causing the underlying call to `self.CurrentRepository()` to fail.

A custom package which calls `Rlocation()` shouldn't be required to provide the `source_repo=` parameter or else run the risk of error due to being found outside of the runfiles directory due to the configuration of `sys.path`.

## 🔬 Minimal Reproduction

`MODULE.bazel`

```
module(name = "reproducer")

bazel_dep(
name = "rules_python",
version = "1.9.0"
)

python = use_extension("@rules_python//python/extensions:python.bzl", "python")
python.toolchain(
python_version = "3.14",
)
```

`BUILD.bazel`

```
load("@rules_python//python:py_binary.bzl", "py_binary")
load("@rules_python//python:py_library.bzl", "py_library")

py_binary(
name = "binary",
srcs = ["binary.py"],
deps = [":library"],
visibility = ["//visibility:public"],
)

py_library(
name = "library",
srcs = ["library/__init__.py"],
deps = ["@rules_python//python/runfiles"],
visibility = ["//visibility:public"],
)
```

`binary.py`

```
from library import rlocation_wrapper

if __name__ == "__main__":
rlocation_wrapper()
```

`library/__init__.py`

```
from python.runfiles import Runfiles

def rlocation_wrapper() -> None:
runfiles_obj = Runfiles.Create()
print(runfiles_obj.Rlocation("library/__init__"))
```

In order to have `sys.path` find the package outside the runfiles directory, we can set the `PYTHONPATH` environment variable to the outer folder containing the package.
To reproduce the failure, run:
```
export PYTHONPATH=/home//
bazelisk run //:binary
```

## 🔥 Exception or Error

```
ValueError: /home///library/__init__.py does not lie under the runfiles root /home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles
```

Full traceback:
```
Traceback (most recent call last):
File "/home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles/_main/_binary_stage2_bootstrap.py", line 537, in
main()
~~~~^^
File "/home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles/_main/_binary_stage2_bootstrap.py", line 531, in main
_run_py_path(main_filename, args=sys.argv[1:])
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles/_main/_binary_stage2_bootstrap.py", line 319, in _run_py_path
runpy.run_path(main_filename, run_name="__main__")
~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "", line 287, in run_path
File "", line 98, in _run_module_code
File "", line 88, in _run_code
File "/home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles/_main/binary.py", line 4, in
rlocation_wrapper()
~~~~~~~~~~~~~~~~~^^
File "/home///library/__init__.py", line 6, in rlocation_wrapper
print(runfiles_obj.Rlocation("library/__init__"))
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
File "/home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles/rules_python+/python/runfiles/runfiles.py", line 306, in Rlocation
source_repo = self.CurrentRepository(frame=2)
File "/home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles/rules_python+/python/runfiles/runfiles.py", line 404, in CurrentRepository
raise ValueError(
...<3 lines>...
)
ValueError: /home///library/__init__.py does not lie under the runfiles root /home//.cache/bazel/_bazel_/0123456789abcdef0123456789abcdef/execroot/_main/bazel-out/k8-fastbuild/bin/binary.runfiles
```

## 🌍 Your Environment

**Operating System:**

```
Ubuntu 24.04.4 LTS
```

**Output of `bazelisk version`:**

```
bazel 9.1.0
```

**Rules_python version:**

```
1.9.0
```

Start with python/runfiles/runfiles.py, especially Rlocation() and CurrentRepository(), at the linked lines, then run the provided minimal reproduction with PYTHONPATH set outside the runfiles directory. Done means the custom package can call Rlocation() without source_repo and the reproduction no longer raises the shown ValueError.

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