Export always includes common dev and extras requirements

• 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: macOS Mojave 10.14.3

• Poetry version: 1.0.5

• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/shparki/36f5cabc9d99f10cc511eb0e186f7473

Issue

This issue seems pretty simple at the front but has caused me a bunch of grief.
TL;DR: No requirements are technically specified, and yet when I perform a basic poetry export -f requirements.txt --without-hashes > requirements.txt the following is set in the requirements.txt:

botocore==1.15.16
docutils==0.15.2
jmespath==0.9.5
python-dateutil==2.8.1
s3transfer==0.3.3
six==1.14.0
urllib3==1.25.8; python_version != "3.4"

with the folloing pyproject.toml:

[tool.poetry]
name = "my-project"
version = "0.1.0"
description = ""
authors = ["Your Name <you@example.com>"]

[tool.poetry.dependencies]
python = "^3.7"
awscli = {version = "^1.18.16", optional = true}

[tool.poetry.dev-dependencies]
boto3 = "^1.12.16"

[tool.poetry.extras]
"aws" = ["awscli"]

[build-system]
requires = ["poetry>=0.12"]
build-backend = "poetry.masonry.api"

I have deleted the pyproject.toml and re-performed poetry update in an attempt to create a fresh poetry.lock, and yet it keeps outputting these requirements. After some experimenting, I believe this is due to those being common requirements between dev and extras. Although those shouldn't be included in a basic export, somehow their common requirements are included. I confirmed this by commenting out the extras and updating and then commenting out the dev requirements and updating and both times the requirements.txt is empty.

I believe this functionality is a bug rather than a feature request due to the inclusion of the --dev and --extras parameters in poetry export to specify all dev/extras requirements to be included.

Steps to Reproduce

1. Create a blank project
2. Define some dependencies as 'dev' and none for core dependencies(note, if you export requirements.txt at this point, it will be empty; for this step, I did boto3 in testing)
3. Define some extra dependencies that have common dependencies as some of the dev dependencies. (for this step, I did awscli for testing)
4. Export core requirements. It should be empty, but in reality, it will contain common dependencies between dev and extras (in testing, mine included botocore, which I knew was a common dependency for boto3 and awscli).

Reproduce the issue with the provided pyproject.toml and poetry export -f requirements.txt --without-hashes, then compare exports with the dev and extras options. Done means a core-only export is empty when only dev and optional dependencies exist, while explicitly requested dev or extras dependencies still export their required packages.