Rename mixin protos and add a check for undefined mixin name

test/protos/mixins.proto

 syntax = "proto2";
 
 import "dart_options.proto";
 
-option (dart_options.imported_mixins) = {
+option (dart_options.imports) = {
   mixins: [{
     name: "Mixin1"
     import_from: "package:protoc_plugin/testing/mixins.dart"
   },
   {
    name: "Mixin2"
    import_from: "package:protoc_plugin/testing/mixins.dart"
   },
   {
     name: "Mixin3"
@@ skipping 16 matching lines @@
   option (dart_options.mixin) = "Mixin2";
 
   optional string overridden_has_method = 3;
 }
 
 message Mixin3PB {
   option (dart_options.mixin) = "Mixin3";
 
   optional string interface_string = 1;
 }