chalice package doesn't handle git repos referenced in requirements.txt properly

### Description
When using `chalice package` to package my app I expect it to respect the git repos referenced in the `requirements.txt` file. Instead it uses the version of the installed package to fetch a wheel. This makes it impossible to reference commits in unreleased versions or forks of published python packages.

### Steps To Reproduce
1. Create a `requirements.txt` file that includes a reference to a git repo (see end of this bug report for an example)
1. Run `chalice package`
1. Extract the zip file
1. Check the contents of `github3/repos/commit.py`
1. Notice that the `associated_pull_requests` method isn't there
1. Swear

This was tested with Chalice 1.18.1 on Python 3.7 with the build running on CodeBuild

### Expected Result
Instead of pulling the wheel, chalice will pull the code from the git repo as instructed by the `requirements.txt` file. This would also result in the `associated_pull_requests` method being present in `github3/repos/commit.py`

### requirements.txt
```txt
-e git+ssh://git@github.com/sigmavirus24/github3.py@197173b5669b189f94c433261497cb652f1db1a9#egg=github3
semantic-version==2.8.5
```

Start by reproducing the issue with the provided requirements.txt and chalice package on the stated Python 3.7 setup. Inspect the generated zip and compare github3/repos/commit.py with the referenced git commit. Done means packaging uses the git repository revision, including associated_pull_requests, rather than a wheel from the installed package.

Written by the indexing model from the issue text.