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

Start with the existing `pip.dep` documentation and compare it with the current `pip.parse` and `requirements.txt` flow in `MODULE.bazel`. Done means dependencies and version requirements can be declared entirely with `pip.dep`, with concrete dependencies resolved without a requirements file.

索引モデルが issue の本文から書いたものです。