URL based dependencies don't generate a hash

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Mac OS X 10.14.6
• Poetry version: 1.0.3
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/lucaskjaero/76424e7ad977613eab9d65623f502bfc

Issue

When adding a normal python dependency, poetry will generate hashes of the downloaded files to put in the lockfile. These can then be given to pip to create reproducible builds. When adding a url dependency, these hashes are not created, and pip cannot successfully install dependencies.

Steps to reproduce

1. poetry add https://github.com/explosion/spacy-models/releases/download/en_core_web_sm-2.2.0/en_core_web_sm-2.2.0.tar.gz
2. poetry export -f requirements.txt | /venv/bin/pip install -r /dev/stdin

Start by reproducing the issue with the provided poetry add URL and poetry export -f requirements.txt | /venv/bin/pip install -r /dev/stdin commands. Trace how URL dependencies are handled during export; done means the exported requirement includes a usable hash and pip can install it successfully.