Enable `pip.dep` to be used for concrete dependency management

# 🚀 feature request

### Relevant Rules

This is an extension of the existing `pip.dep` feature: https://rules-python.readthedocs.io/en/latest/api/rules_python/python/extensions/pip.html#pip.dep.

### Description

`pip.dep` today is only used for abstract dependency management. The concrete dependencies still must come through `pip.parse` and a `requirements.txt` file.

### Describe the solution you'd like

It would be nice to be able to specify dependencies / version requirements entirely in MODULE.bazel using `pip.dep`. This is similar to how the `crate_universe` feature works in `rules_rust`.

### Describe alternatives you've considered

N/A

从现有的 `pip.dep` 文档开始，将其与 `MODULE.bazel` 中当前的 `pip.parse` 和 `requirements.txt` 流程进行比较。完成标准是：依赖项和版本要求可以完全使用 `pip.dep` 声明，并且无需 requirements 文件即可解析具体依赖项。

由索引模型根据 Issue 内容生成。