Poetry export does not include setuptools even when it is a direct dependency.

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- **OS version and name**: Fedora 31

- **Poetry version**: 1.0.0b4 (at the time of writing b5 is not on PyPi, will retry later on)

- **Link of a [Gist](https://gist.github.com/) with the contents of your pyproject.toml file**: https://gist.github.com/apollo13/1d0c72978531d3a5688f1e3847fa6fe6



## Issue

Using the `pyproject.toml` file from the Gist and executing `poetry lock` I get the following lockfile:

```

[[package]]

category = "main"

description = "WSGI HTTP Server for UNIX"

name = "gunicorn"

optional = false

python-versions = ">=3.4"

version = "20.0.0"



[package.dependencies]

setuptools = ">=3.0"



[package.extras]

eventlet = ["eventlet (>=0.9.7)"]

gevent = ["gevent (>=0.13)"]

setproctitle = ["setproctitle"]

tornado = ["tornado (>=0.2)"]



[metadata]

content-hash = "20c8a1cc7d149b3ad58ca5063dfd37654f03ba65ee8b484d2faefe7ce28b7daf"

python-versions = "^3.7"



[metadata.files]

gunicorn = [

    {file = "gunicorn-20.0.0-py2.py3-none-any.whl", hash = "sha256:0806b5e8a2eb8ba9ac1be65d7b743ec896fc25f5d6cb16c5e051540157b315bb"},

    {file = "gunicorn-20.0.0.tar.gz", hash = "sha256:ef69dea4814df95e64e3f40b47b7ffedc6911c5009233be9d01cfd0d14aa3f50"},

]

```

setuptools is correctly discovered as dependency of gunicorn (see https://github.com/benoitc/gunicorn/blob/94ab2091173c6037b504f94e56f4e88816d540bf/setup.py#L71-L77 -- it does indeed require it), but the lockfile *does not contain* any hashes for setuptools.

If I now export to requirements.txt the file looks like this:

```

gunicorn==20.0.0 \

    --hash=sha256:0806b5e8a2eb8ba9ac1be65d7b743ec896fc25f5d6cb16c5e051540157b315bb \

    --hash=sha256:ef69dea4814df95e64e3f40b47b7ffedc6911c5009233be9d01cfd0d14aa3f50

```

Which is not installable with `pip install --require-hashes --force-reinstall -r requirements.txt` (note the `--require-hases` there):

```

Collecting gunicorn==20.0.0 (from -r requirements.txt (line 1))

  Using cached https://files.pythonhosted.org/packages/60/0d/3dbda0324f5bf007f3274e5ea09f0f3bcbf0ca01a75b80ff4f1ff9f8ecfd/gunicorn-20.0.0-py2.py3-none-any.whl

Collecting setuptools>=3.0 (from gunicorn==20.0.0->-r requirements.txt (line 1))

ERROR: In --require-

[]