Bad error when referring to local repo by git url

[roberth]

Describe the bug

Easy mistake:
inputs.url = "file:///home/user/src/foo";

       … while fetching the input 'file:///home/user/src/foo'

       error: Failed to open archive (Unrecognized archive format)

(Solution: use git+file://)

Steps To Reproduce

Easier reproducer:

$ nix build file://$HOME/src/nixpkgs#hello
error:
       … while fetching the input 'file:///home/user/src/nixpkgs'

       error: Failed to open archive (Unrecognized archive format)

Expected behavior

Error message contains relevant info

• file URLs are for tarballs only
• the path is a directory
• bonus points: provide a suggestion if it's a repo

Metadata

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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