Gazelle python_root directive only adds imports to subfolders

# 🐞 bug report

### Affected Rule

Gazelle plugin target generation

### Is this a regression?

No

### Description

python targets that do not share root with the workspace needs `imports = [...]` statements added for imports to work correctly. The gazelle plugin supports this through adding `# gazelle:python_root` directives in `BUILD` files that are at the root of python trees. This works great for source files in subfolders, they have attributes like `imports = ["../.."],` added. However the sources files in the root folder itself does not get any imports attribute, while they need to have `imports = ["."]` to work correctly.

## 🔬 Minimal Reproduction

## 🔥 Exception or Error

Target py_binary file fails on run due to missing import.

## 🌍 Your Environment

**Operating System:**

  

     Ubuntu 24.10

  

**Output of `bazel version`:**

  

    Bazelisk version: v1.25.0

    Build label: 8.0.1

    Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer

    Build time: Fri Jan 17 19:16:16 2025 (1737141376)

    Build timestamp: 1737141376

    Build timestamp as int: 1737141376

  

**Rules_python version:**

  

     bazel_dep(name = "rules_python", version = "1.1.0")

     bazel_dep(name = "rules_python_gazelle_plugin", version = "1.1.0")

  

**Anything else relevant?**

首先跟踪 Gazelle 插件针对包含 `# gazelle:python_root` 指令的 BUILD 文件生成目标的过程，重点关注根目录级别的 Python 源文件和生成的 `py_binary` 目标。比较根目录目标与子目录目标，并验证生成完成后是否在需要的位置添加了 `imports = ["."]`，然后确认该目标能够运行且不会出现缺少导入的失败。

由索引模型根据 Issue 内容生成。