poetry-core 1.0.1 release breaks installation poetry via pip

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- **OS version and name**: `ubuntu-latest` at GHA

- **Poetry version**: 1.1.4



## Issue



Unfortunately, `poetry-core==1.0.1` release breaks `poetry`, when installed via pip.



### Steps to reproduce



1. Create virtual environment for test: `python3 -m venv /path/to/venv`

2. Install poetry within venv: `/path/to/venv/bin/python3 -m pip install poetry==1.1.4`

3. Attempt to get help: `/path/to/venv/bin/poetry --help`

4. Results in error below,



```

Traceback (most recent call last):

  File "bin/poetry", line 5, in <module>

    from poetry.console import main

  File "/path/to/venv/lib/python3.8/site-packages/poetry/console/__init__.py", line 1, in <module>

    from .application import Application

  File "/path/to/venv/lib/python3.8/site-packages/poetry/console/application.py", line 7, in <module>

    from .commands.about import AboutCommand

  File "/path/to/venv/lib/python3.8/site-packages/poetry/console/commands/__init__.py", line 4, in <module>

    from .check import CheckCommand

  File "/path/to/venv/lib/python3.8/site-packages/poetry/console/commands/check.py", line 2, in <module>

    from poetry.factory import Factory

  File "/path/to/venv/lib/python3.8/site-packages/poetry/factory.py", line 16, in <module>

    from .packages.locker import Locker

  File "/path/to/venv/lib/python3.8/site-packages/poetry/packages/__init__.py", line 2, in <module>

    from .locker import Locker

  File "/path/to/venv/lib/python3.8/site-packages/poetry/packages/locker.py", line 28, in <module>

    from poetry.core.packages.package import Dependency

ImportError: cannot import name 'Dependency' from 'poetry.core.packages.package' (/path/to/venv/python3.8/site-packages/poetry/core/packages/package.py)

```



### How to fix



Install `poetry-core==1.0.0` version, when installing poetry via pip.



### Reason



`poetry.core.packages.package` is not a source of `Dependency` class, it only imports it for type checking: https://github.com/python-poetry/poetry-core/blob/master/poetry/core/packages/package.py#L27



**ps.** Yes, I understand, that installing poetry via pip is not an official installation method. But at a moment it is an ea

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