[BUG] Using dart-dio generator with custom source folder produce invalid import statements on Windows

Description

When using dart-dio generator with custom source folder on Windows produces invalid import statements

import 'package:openapi/test\source/auth/api_key_auth.dart';
import 'package:openapi/test\source/auth/basic_auth.dart';
import 'package:openapi/test\source/auth/bearer_auth.dart';
import 'package:openapi/test\source/auth/oauth.dart';
import 'package:openapi/test\source/api/auth_endpoint_api.dart';

should be

import 'package:openapi/test/source/auth/api_key_auth.dart';
import 'package:openapi/test/source/auth/basic_auth.dart';
import 'package:openapi/test/source/auth/bearer_auth.dart';
import 'package:openapi/test/source/auth/oauth.dart';
import 'package:openapi/test/source/api/auth_endpoint_api.dart';

so '\' must be replaced by '/'

openapi-generator version

from master branch - 7.2.0-SNAPSHOT

Generation Details

generator: dart-dio
additional config:
sourceFolder: test/source

Steps to reproduce

    @Test
    public void generateModel_withSourceFolder_expectValidImport() throws Exception {
        final var inputSpec = "src/test/resources/3_0/petstore.json";

        final var output = Files.createTempDirectory("test").toFile();
        output.deleteOnExit();

        Assert.assertTrue(new File(inputSpec).exists());

        final var configurator = new CodegenConfigurator()
                .setGeneratorName("dart-dio")
                .addAdditionalProperty(CodegenConstants.SOURCE_FOLDER, "test/source")
                .setInputSpec(inputSpec)
                .setOutputDir(output.getAbsolutePath());

        final var clientOptInput = configurator.toClientOptInput();
        new DefaultGenerator().opts(clientOptInput).generate();

        var apiFile = new File(output, "/lib/test/source/api.dart");
        Assert.assertTrue(apiFile.exists());
        Assert.assertTrue(Files.readString(apiFile.toPath()).contains("import 'package:openapi/test/source/serializers.dart';"));
    }

Suggest a fix

In all dart templates change property sourceFolder to new property importPrefix that will not contains windows folder separator char

Start with the dart-dio generator templates mentioned in the issue and the test method generateModel_withSourceFolder_expectValidImport. Run it on Windows or inspect the generated lib/test/source/api.dart output using petstore.json. Done means imports use forward slashes when sourceFolder is test/source and the test assertion passes.