Make it easier to load wheels from the local file system

# 🚀 feature request

### Relevant Rules

Possibly in `whl_library`, but more likely in `requirements.txt`

### Description

I had created a patched wheel that I wished to test with Bazel and rules_python *before* publishing it to our internal Artifactory package index. There was no easy way to accomplish this.

I eventually worked out that I could test my patched wheel by using [pip2pi](https://pypi.org/project/pip2pi/) to create a local package index on my filesystem, then adding `--index-url=/tmp/pypi/simple` to the beginning of my `requirements.txt`.

See [this writeup](https://github.com/georgevreilly/torch21?tab=readme-ov-file#using-the-patched-wheel-with-rules_python) for full details.

### Describe the solution you'd like

If you have a solution in mind, please describe it.

I think the easiest thing for users is if they can specify a file path in `requirements.txt`; e.g.,

```
sympy==1.12
./tmp/torch-2.1.0+stripe.5-cp38-cp38-manylinux1_x86_64.whl
triton==2.1.0
```

Pip allows this, after all.

### Describe alternatives you've considered

Have you considered any alternative solutions or workarounds?

首先阅读 rules_python 中对 whl_library 和 requirements.txt 的处理，以追踪依赖项条目是如何被解析和解决的。检查现有测试是否覆盖本地软件包源；完成的标准是，requirements.txt 中的 wheel 路径可以被 Bazel 规则使用，而无需发布到外部软件包索引。

由索引模型根据 Issue 内容生成。