Implement PEP-708 - Extend the repository API to mitigate "dependency confusion" attacks

PEP-708

Dependency confusion attacks, in which a malicious package is installed instead of the one the user expected, are an increasingly common supply chain threat. Most such attacks against Python dependencies, including the recent PyTorch incident, occur with multiple package repositories, where a dependency expected to come from one repository (e.g. a custom index) is installed from another (e.g. PyPI).

To help address this problem, this PEP proposes extending the Simple Repository API to allow repository operators to indicate that a project found on their repository “tracks” a project on different repositories, and allows projects to extend their namespaces across multiple repositories.

These features will allow installers to determine when a project being made available from a particular mix of repositories is expected and should be allowed, and when it is not and should halt the install with an error to protect the user.

Start by reading PEP-708 and the linked Simple Repository API specification. The issue names no files or tests; done means extending the repository API to support project tracking and namespace extension as specified, with coverage for the dependency-confusion protections.