versionCheckHook: emit warning when falling back to pname

pkgs/by-name/ve/versionCheckHook/hook.sh

···
     elif [[ ! -z "${NIX_MAIN_PROGRAM-}" ]]; then
         cmdProgram="${!outputBin}/bin/${NIX_MAIN_PROGRAM}"
     elif [[ ! -z "${pname-}" ]]; then
+        echo "versionCheckHook: Package \`${pname}\` does not have the \`meta.mainProgram\` attribute." \
+            "We'll assume that the main program has the same name for now, but this behavior is deprecated," \
+            "because it leads to surprising errors when the assumption does not hold." \
+            "If the package has a main program, please set \`meta.mainProgram\` in its definition to make this warning go away." \
+            "Should the binary that outputs the intended version differ from \`meta.mainProgram\`, consider setting \`versionCheckProgram\` instead." >&2
         cmdProgram="${!outputBin}/bin/${pname}"
     else
         echo "versionCheckHook: \$NIX_MAIN_PROGRAM, \$versionCheckProgram and \$pname are all empty, so" \