Using ES Modules breaks when a directory with the same name as the module exists

If you setup a project with the following structure:

1. `"type": "module"` in package.json
2. A `hello-world.js` file
3. A `hello-world/` folder
4. Use `hello-world.lambdaHandler` as your module/handler
5. Enable verbose mode, e.g. with `ENV AWS_LAMBDA_RUNTIME_VERBOSE 1` in a Docker container using `public.ecr.aws/lambda/nodejs:20`, so you can see the verbose messages

This is the resulting log.

```
START RequestId: 99e7b316-ab71-4bd2-8aed-85df8b0aeaaf Version: $LATEST
2023-12-13T01:13:42.274Z undefined INFO RUNTIME LOADER Try loading as commonjs: hello-world with paths: , /var/task
2023-12-13T01:13:42.275Z undefined INFO RUNTIME LOADER Try loading as commonjs: /var/task/hello-world
2023-12-13T01:13:42.276Z undefined INFO RUNTIME LOADER globalPaths ["/opt/nodejs/node20/node_modules","/opt/nodejs/node_modules","/var/runtime/node_modules","/var/runtime","/var/task","/root/.node_modules","/root/.node_libraries","/var/lang/lib/node"]
2023-12-13T01:13:42.276Z undefined ERROR Uncaught Exception {"errorType":"Runtime.ImportModuleError","errorMessage":"Error: Cannot find module '/var/task/hello-world'\nRequire stack:\n- /var/runtime/index.mjs","stack":["Runtime.ImportModuleError: Error: Cannot find module '/var/task/hello-world'","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"]}
13 Dec 2023 01:13:42,288 [ERROR] (rapid) Init failed error=Runtime exited with error: exit status 129 InvokeID=
2023-12-13T01:13:42.351Z undefined INFO RUNTIME LOADER Try loading as commonjs: hello-world with paths: , /var/task
2023-12-13T01:13:42.352Z undefined INFO RUNTIME LOADER Try loading as commonjs: /var/task/hello-world
2023-12-13T01:13:42.353Z undefined INFO RUNTIME LOADER globalPaths ["/opt/nodejs/node20/node_modules","/opt/nodejs/node_modules","/var/runtime/node_modules","/var/runtime","/var/task","/root/.node_modules","/root/.node_libraries","/var/lang/lib/node"]
2023-12-13T01:13:42.353Z undefined ERROR Uncaught Exception {"errorType":"Runtime.ImportModuleError","errorMessage":"Error: Cannot find module '/var/task/hello-world'\nRequire stack:\n- /var/runtime/index.mjs","stack":["Runtime.ImportModuleError: Error: Cannot find module '/var/task/hello-world'","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"]}
END RequestId: a879dd58-2f3f-458a-9911-dbb92b552019
REPORT RequestId: a879dd58-2f3f-458a-9911-dbb92b552019 Init Duration: 0.29 ms Duration: 156.42 ms Billed Duration: 157 ms Memory Size: 128 MB Max Memory Used: 128 MB
{"errorType":"Runtime.ImportModuleError","errorMessage":"Error: Cannot find module '/var/task/hello-world'\nRequire stack:\n- /var/runtime/index.mjs","trace":["Runtime.ImportModuleError: Error: Cannot find module '/var/task/hello-world'","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"]}
```

# Cause

Before trying to load a module an extensionless import is attempted. Because `fs.existsSync` does not differentiate between a directory and a file it returns true and a `require` on the directory is attempted without any try catch.

https://github.com/aws/aws-lambda-nodejs-runtime-interface-client/blob/7374a4e338fc3b070811d55ee337740f3f6cb382/src/UserFunction.js#L133-L174

https://github.com/aws/aws-lambda-nodejs-runtime-interface-client/blob/7374a4e338fc3b070811d55ee337740f3f6cb382/src/UserFunction.js#L73

# Background

I was working on a project using multiple ESModules based lambda functions. The code was transpiled from TypeScript and because some code needed to be shared between functions I had a `hello-world.js` in the root that was `export * from './dist/hello-world/src/app.js';` to so the lambda function ran off the transpiled code.