Add helpful messages when file:// used as tarball

[fzakaria]

Motivation

When file:// is used accidentally in a flake as the source it is expected to be a tarball by default.
Add some friendlier error messages to either inform the user this is not in fact a tarball or if it's a git directory, let them know they can use git+file.

Please see #12935 for more in depth motivation.

fixes #12935

Validation

** Missing directory **

nix build file://$HOME/code/github.com/does-not-exist
error:
       … while fetching the input 'file:///home/fmzakari/code/github.com/does-not-exist'

       error: tarball '/home/fmzakari/code/github.com/does-not-exist' does not exist.

** directory BUT is git **

nix build file://$HOME/code/github.com/fzakaria/fzakaria.com
error:
       … while fetching the input 'file:///home/fmzakari/code/github.com/fzakaria/fzakaria.com'

       error: tarball '/home/fmzakari/code/github.com/fzakaria/fzakaria.com' is a git repository, not a tarball. Please use `git+file` as the scheme.

** directory BUT is NOT git **

nix build file://$HOME/code/github.com/fzakaria
error:
       … while fetching the input 'file:///home/fmzakari/code/github.com/fzakaria'

       error: tarball '/home/fmzakari/code/github.com/fzakaria' is a directory, not a file.

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