Issue with working serverless-webpack

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As the documentation says, I added this configuration inside my `custom` property in the serverless.yaml

```yaml
localstack:
stages:
# list of stages for which the plugin should be enabled
- local
- development
host: http://localhost # optional - LocalStack host to connect to
edgePort: 4566 # optional - LocalStack edge port to connect to
autostart: true # optional - Start LocalStack in Docker on Serverless deploy
networks: #optional - attaches the list of networks to the localstack docker container after startup
- host
- overlay
lambda:
# Enable this flag to improve performance
mountCode: true # specify either "true", or a relative path to the root Lambda mount path
```

The `webpack` config is this for the serverless:

```yaml
webpack:
webpackConfig: ./webpack.config.js
includeModules: true
excludeFiles: tests/**/*.{js,ts}
keepOutputDirectory: true
```

This is the webpack configuration:

```js
const path = require('path');
const slsw = require('serverless-webpack');
const nodeExternals = require('webpack-node-externals');

module.exports = {
entry: slsw.lib.entries,
target: 'node',
mode: slsw.lib.webpack.isLocal ? 'development' : 'production',
optimization: {
minimize: false,
},
plugins: [],
performance: {
// Turn off size warnings for entry points
hints: false,
},
externals: [nodeExternals()],
module: {
rules: [
{
test: /\.ts$/,
use: 'ts-loader?configFile=tsconfig.build.json',
exclude: [/node_modules/, /tests/],
},
],
},
resolve: {
extensions: ['.ts', '.js'],
},
output: {
libraryTarget: 'commonjs2',
path: path.join(__dirname, '.webpack'),
filename: '[name].js',
},
cache: {
type: 'filesystem',
},
};
```

And I'm getting this error:

```
Execution environment startup failed: {"errorType":"Runtime.ImportModuleError","errorMessage":"Error: Cannot find module 'a-handler'\nRequire stack:\n- /var/runtime/index.mjs","trace":["Runtime.ImportModuleError: Error: Cannot find module 'a-handler'","Require stack:","- /var/runtime/index.mjs"," at _loadUserApp (file:///var/runtime/index.mjs:1087:17)"," at async UserFunction.js.module.exports.load (file:///var/runtime/index.mjs:1119:21)"," at async start (file:///var/runtime/index.mjs:1282:23)"," at async file:///var/runtime/index.mjs:1288:1"]}
```

And it looks like is because the handler path is incorrect, it says this

` '_HANDLER': '.webpack/service/src/a-dir/a-handler.handle'`

But when webpack package the code is located in `.webpack/a-dir/a-handler.handle`, what would be the solution for that, I'm kind of lost, should I update the `output` path?

Start with the serverless.yaml handler configuration and webpack.config.js, then compare the generated .webpack paths with the _HANDLER value shown in the error. Check how the plugin and serverless-webpack derive the packaged handler path. Done means the deployed handler path matches the generated .webpack/a-dir/a-handler.handle location and the LocalStack invocation starts successfully.

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