Poetry shell command does not start shell

• 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) (Not applicable).

• OS version and name: macos 10.15.6

• Poetry version: 1.0.10

• Link of a Gist with the contents of your pyproject.toml file: N/A

Issue

The basic issue is that poetry shell cannot be invoked within a virtual environment. Based on how this runs, I think the expectation is that we install poetry into the system.

My expectation is that I can invoke a poetry shell and drop into a standard virtual environment (i.e. a subshell that runs similar to vex).

Reproduceable steps:

`which python3` -m venv ~/.test-venv
. ~/.test-venv/bin/activate
pip install --upgrade pip poetry
deactivate
~/.test-venv/bin/poetry shell

The error will be something like:

Skipping virtualenv creation, as specified in config file.
Virtual environment already activated: ~/.test-venv

However, the virtual environment is not active.