fix: add fallback for finding bundled library when importlib.metadata unavailable

[Wheest]

Summary

When running in sandboxed environments like Bazel (with rules_python), importlib.metadata.files() may return None or fail to locate files correctly, even though the bundled libspatialindex library exists in the wheel.

This PR adds a fallback that searches for the bundled library relative to the package directory (rtree.libs/), which works regardless of metadata availability.

Problem

In Bazel's sandboxed test environment:

OSError: Could not load libspatialindex_c library

The library exists at rtree.libs/libspatialindex-*.so but importlib.metadata.files("rtree") returns None because Bazel doesn't properly expose package metadata.

Solution

After the existing importlib.metadata lookup, add a simple fallback:

libs_dir = _cwd.parent / "rtree.libs"
if libs_dir.exists():
    for so_file in libs_dir.glob("libspatialindex*.so"):
        _candidates.insert(1, so_file)
        break

This uses the already-defined _cwd (Path(__file__).parent) to locate the library relative to the package, which works in any environment.

Testing

• Added test that mocks importlib.metadata.files() returning None
• Verified fix works in Bazel environment (commit 1 fails, commit 2 passes)