warn about unknown flake url paramaters

[lolbinarycat]

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.

Priorities

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[bryanhonof]

Are there currently store types that allow for a query part without it being a key value pair?
I don't recall ever encountering one where the equal sing was missing, and if it was used as some sort of flag, it's more often than not key=val.

If the above is the case, maybe we should throw an error if someone tries to use a store URL that includes the key part in the query, but doesn't assign a value.

[bryanhonof]

It looks like the type is even a map of string to string.

nix/src/libutil/url.cc

Line 76 in af26fe3

std::map<std::string, std::string> decodeQuery(const std::string & query)