firebase-functions do not deploy on node 10 if the source directory is named "firebase"

### Related issues

### [REQUIRED] Version info

**node:** 10.21.0

**firebase-functions:** 3.6.1

**firebase-tools:** 8.5.0

**firebase-admin:** 8.10.0

### [REQUIRED] Test case

setting `functions.source` in `firebase.json` keeps the functions from being deployed.
Having the functions in the `functions` directory and no custom source folder defined, everything deploys like it should.

Repo with mcve: https://github.com/jbreckel/firebase-functions-source-repro

### [REQUIRED] Steps to reproduce

1. clone repo (it's a blank typescript project from `firebase init`)
2. run `firebase deploy --only functions`

### [REQUIRED] Expected behavior

firebase reads the `functions.source` setting from `firebase.json` and uploads the functions in that folder.

e.g:
```
$ firebase deploy --only functions

=== Deploying to 'project-id'...

i deploying functions

✔ functions: Finished running predeploy script.
i functions: ensuring required API cloudfunctions.googleapis.com is enabled...
i functions: ensuring required API cloudbuild.googleapis.com is enabled...
✔ functions: required API cloudbuild.googleapis.com is enabled
✔ functions: required API cloudfunctions.googleapis.com is enabled
i functions: preparing functions directory for uploading...
i functions: packaged functions (41.99 KB) for uploading
✔ functions: functions folder uploaded successfully
i functions: creating Node.js 10 function helloWorld(us-central1)...
✔ functions[helloWorld(us-central1)]: Successful create operation.
Function URL (helloWorld): https://us-central1-project-id.cloudfunctions.net/helloWorld

✔ Deploy complete!
```

### [REQUIRED] Actual behavior

firebase does not find any functions to upload
```
$ firebase deploy --only functions

=== Deploying to 'project-id'...

i deploying functions

✔ functions: Finished running predeploy script.
i functions: ensuring required API cloudfunctions.googleapis.com is enabled...
i functions: ensuring required API cloudbuild.googleapis.com is enabled...
✔ functions: required API cloudbuild.googleapis.com is enabled
✔ functions: required API cloudfunctions.googleapis.com is enabled
i functions: preparing firebase directory for uploading...

✔ Deploy complete!

Project Console: https://console.firebase.google.com/project/project-id/overview
```

### Were you able to successfully deploy your functions?

no error, just no functions found