Generated yarn.lock from package-lock.json

This commit uses the command `yarn import` to generate a `yarn.lock`
file from an existing `package-lock.json`. Unfortunately, a
[known bug](yarnpkg/yarn#2240) exists. When
one runs `yarn install`, yarn **does not** actually install the
dependencies in `node_modules`; instead it only creates a "yarn
integrity check file."

To actually install the dependencies, one must run:

- `yarn import`
- `rm -rf node_modules`
- `yarn install`
- `rm -rf package-lock.json` (to avoid future warnings about multiple
  lock files).