Default repositories for github: fetcher

[max-privatevoid]

Is your feature request related to a problem? Please describe.
Projects on GitHub may opt to use a naming structure in which the GitHub organization's name is the same as the main repository's name. This repetition can lead to unnecessarily long names when using github: flake references, especially on the CLI.

Describe the solution you'd like
As a shortcut, I think Nix should be able to parse github:my-project to be equivalent to github:my-project/my-project, such that nix run github:my-project#package1 will download from the repository github.com/my-project/my-project and run the package attribute package1.

Describe alternatives you've considered
The obvious alternative is the flake registry, but that comes with its own drawbacks:

• need to request inclusion of my project in the registry
• can only be used when the user has registries enabled and uses the default registry
  • the flake registry represents an inclusion of external state, which may be against the desires of strongly reproducibility-minded individuals

Additional context
This feature request resulted from the discussion in nixified-ai/flake#21

Priorities

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[SuperSandro2000]

Flake URIs have already enough parts you need to know like the query strings and this would break backwards compatibility.