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.

Start by reproducing the documented macOS command sequence and inspect the poetry shell command handling. Verify how it behaves when run from an already activated virtual environment. Done means poetry shell opens the expected subshell instead of reporting that the virtual environment is already active.