Stabilize the (non-flakes) installable syntax

[thufschmitt]

Part of #7701.

The syntax for non-flakes installables isn't optimal, and probably not exactly what we want to stabilize.
After considering various options and asking about them, we settled on the second one, namely:

• By default, positional arguments are not allowed on the command-line (or parsed as flake references when flakes are enabled)
• After encountering --file /some/file.nix, the CLI parser interprets every positional argument as an attribute path in import /some/file.nix until the next --file/--expr(/--flake) argument
• After encountering --expr e, the CLI parser interprets every positional argument as an attribute path in the nix expression e until the next --file/--expr(/--flake) argument
• (After encountering --flake, the CLI parser interprets every positional argument as a flake reference until the next --file/--expr/--flake argument)

with the parenthesized bits corresponding to the behaviour with flakes enabled.

This new behaviour is open for implementation to anyone who would want to give it a stab.

[Ericson2314]

(See also #10047)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-02-19-nix-team-meeting-minutes-126/40308/1

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-55/40996/1