Motivation for this change

buildRustCrate has a handy include helper, that only imports those whitelisted
files and folders to the store.

However, the function's matching logic is broken and includes all files,
regardless of whether or not they're whitelisted, as long as the whitelist
contains at least one name (regardless of whether that name exists). This is
because it doesn't take into account that
lib.strings.removePrefix "foo" "bar" == "bar" (that is, paths that don't match
the prefix are passed straight through).

I've tested that include now does what you'd expect, but not that packages actually used it correctly in the first place.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.