support for gathering local pip packages on deploy.

I have a local python package installed via pip on my machine. `chalice deploy` does not work after doing a `pip freeze > requirements.txt`. Reading the documentation, the chalicelibs/ and vendor/ folders are my only options. The problem comes from building multiple chalice projects. I want a way to share code between projects without having to manually drag and drop everything into chalicelib/ or vendor/ for each deploy.

`chalice local` works without issue after configuring my package locally via pip.

My req.txt that doesnt work.
```txt
pymongo
-e git+https://github.com/company/somesharedlib.git@d0fcd1e0cb0b43ac4cbf1d587cd7e62814245549#egg=somesharedlib
```

```python
from chalice import Chalice
from chalicelib.models import SomeModel
# I would like to do this instead
from somesharedlib.models import SomeModel

app = Chalice(app_name='temp-svc')

@app.route('/')
def index():
log = PresenceStatusLog('test')
return {'hello': 'world'}
```

It would be great to have a simple feature that allows for say pointing the chalicelib/ directory at some other directory on your local machine at the very least so people could share code between projects. Currently writing a bash script to copy the contents of that directory each time i deploy.

Start by reviewing how `chalice deploy` handles `requirements.txt`, `chalicelib/`, and `vendor/`; compare this with the working `chalice local` behavior and the shown editable Git dependency. The issue names no source files or tests. Done means a repeatable deployment path for sharing local pip packages across multiple Chalice projects.

Written by the indexing model from the issue text.