Support running an app from a subdirectory

There is currently no supported way to run a node app from a directory within your git repo. You can work around it most of the time by using `git subtree push` but that wouldn't work if you have shared code outside of your directory.

A super common situation is to have the following repo structure:

```
├── shared-code
├── server
└── frontend-app
```

Being able to direct the buildpack to look in `/server` instead of `/` would be helpful for cases like this.

Start by reviewing the buildpack entry points and existing tests that determine the application root, then inspect how the repository currently assumes the root directory. Confirm the expected behavior for a repository containing shared-code, server, and frontend-app, and treat the issue discussion as context for the unresolved design.

Written by the indexing model from the issue text.