Is your feature request related to a problem? Please describe.
recently i made a typo and accidentally tried to install a package from github:nixos/nixpkgs?revnixpkgs-unstable (notice the lack of =), and nix just built the whole thing successfully (i assume using the master branch instead)

Describe the solution you'd like
a warning should be displayed if there are unknown keys in a url-style flake reference (eg. "warning: unknown flake attribute XXX, check your spelling or update nix")

at least for github: or flake: urls, i assume for git+http url unrecognized paramaters are just passed to the http server.

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