Merge unreleased changes from master into develop

Previously, the editable builder contained a regression that only
included poetry root when generating .pth files. This change ensure
that all package or module sources are included as per configuration.

Resolves: #2657

This is a workaround for a bug in 1.0.9 causing a recursion error when
installing dependencies.

# Conflicts:
#	.github/workflows/main.yml
#	CHANGELOG.md
#	docs/docs/cli.md
#	get-poetry.py
#	poetry/packages/package.py
#	poetry/spdx/__init__.py
#	poetry/utils/env.py
#	poetry/vcs/git.py
#	tests/packages/test_package.py
#	tests/spdx/test_main.py
#	tests/utils/test_env.py

Until now poetry used the built-in venv module available in Python 3.
This has presented a few concerns. Chief of which has been
inconsistent environment setup. This includes, but is not limited to,
pip version used by the `ensurepip` module which is in turn used by the
built-in `venv.EnvBuilder`. Additionally, the `virtualenv` package
allows for faster environment creation. This can also allow us to, going
forward, drop ad-hoc code required to inspect and manage environments.

Previously, environment run method handled shell commands incorrectly
for windows platforms. This change ensures that command list to shell
conversion occurs after shell option is enforced.

Release 1.1.0a3

* Improve the way packages are installed

* Add support for parallelized operations

* Improve the Chooser class

* Make the new installer the default

* Add an Authenticator class to be able to download from protected urls

* Adapt code and tests to latest changes

* Update lock file and some dependencies

* Improve installations information and caching

* Make the final adjustments and tests for the new installer

* Use the preview version of Poetry for the CI

* Rename Executor.run() to Executor.run_pip()

* Gracefully handle errors when executing operations

* inspection: improve handling of sdist packages

This change ensures that inspection does not fail pre-maturely if
source does not contain a setup.py and no dependencies were
discovered.

Resolves: #2603

* inspection: add tests for package info discovery

Resolves: #2597

Ensure path requirement is converted to posix string.

Relates-to: #2398

Release 1.1.0a2

Add error handling for malfoemed author

Python version option (`--python`) for `init` script.

Provide hints when an invalid license id is input

Adds ways to specify a package to poetry add --help section.

Link with 'www' was being redirected wrong to 'https://spdx.org//licenses/'

These small changes allow `poetry shell` to properly activate a python virtual environment when the user's shell is `tcsh`.

Keep empty in-project venv dir when recreating venv to allow for docker volumes