Override a requirement with a Bazel target

# 🚀 feature request

### Relevant Rules

`load("@rules_python//python:pip.bzl", "pip_install")`

### Description

We'd like to be able to override a `requirement` with a Bazel target. This is basically the `rules_python` equivalent of `rules_jvm_external`'s [`override_targets` directive](https://github.com/bazelbuild/rules_jvm_external#overriding-generated-targets).

Consider the case where we're using grpc and protobuf from Bazel, and an external python library depends on `protobuf==3.13.0` (or whatever). I'd like to replace that dependency with `"@com_google_protobuf//:protobuf_python"` instead, so I'm not mixing pypi and Bazel versions of `protobuf`.

### Describe the solution you'd like

I like the way that `rules_jvm_external` has done it.

### Describe alternatives you've considered

ø

Start with the pip_install rule described in the issue and compare its dependency handling with rules_jvm_external's override_targets directive. The change is done when a Python requirement can be replaced by the specified Bazel target without mixing the corresponding PyPI and Bazel dependencies.

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