[Gazelle][proto] How to automatically resolve protobuf target

# 🐞 bug report

### Affected Rule

gazelle resolve

### Is this a regression?
No

### Description

I have a python file with a proto definition imported. When I try to use gazelle to auto generate py_library target, it fails to validate dependency. I look into some code in bazel_gazelle/resolve/index.go, no proto target defined in there.

## 🔬 Minimal Reproduction

  

import a.b.c_pb2

  

and then run bazel run :py_gazelle

## 🔥 Exception or Error

  

gazelle: ERROR: failed to validate dependencies for target "//:check": "a.b.c_pb2" at line 11 from "check.py" is an invalid dependency: possible solutions:

	1. Add it as a dependency in the requirements.txt file.

	2. Instruct Gazelle to resolve to a known dependency using the gazelle:resolve directive.

	3. Ignore it with a comment '# gazelle:ignore a.b.c_pb2' in the Python file.


"a.b" at line 11 from "check.py" is an invalid dependency: possible solutions:

	1. Add it as a dependency in the requirements.txt file.

	2. Instruct Gazelle to resolve to a known dependency using the gazelle:resolve directive.

	3. Ignore it with a comment '# gazelle:ignore a.b' in the Python file.

"a" at line 11 from "check.py" is an invalid dependency: possible solutions:

	1. Add it as a dependency in the requirements.txt file.

	2. Instruct Gazelle to resolve to a known dependency using the gazelle:resolve directive.

	3. Ignore it with a comment '# gazelle:ignore a' in the Python file.

  

## 🌍 Your Environment

**Operating System:**

  

ubuntu 20.04

  

**Output of `bazel version`:**

  

Bazelisk version: v1.16.0

Build label: 6.2.1

Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar

Build time: Fri Jun 2 16:59:58 2023 (1685725198)

Build timestamp: 1685725198

Build timestamp as int: 1685725198

  

**Rules_python version:**

  

0.25

  

**Anything else relevant?**
rules_python_gazelle_plugin version
0.28

从 bazel_gazelle/resolve/index.go 开始，使用 `bazel run :py_gazelle` 通过所示的 `import a.b.c_pb2` 示例复现该失败。跟踪 `gazelle resolve` 如何验证 Python 依赖，并确定应如何识别 protobuf 生成的 target；完成标准是该示例能够生成有效的 `py_library` 依赖，而无需手动的 resolve 指令。

由索引模型根据 Issue 内容生成。