Implement a new bootstrapping method

[sdispater]

This method introduces a new script install-poetry.py which implements a new way to bootstrap Poetry. The advantages of this new method are:

• It no longer relies on the prebuilt release packages of Poetry (see https://github.com/python-poetry/poetry/releases/tag/1.1.4 for an example) which had the issues of having frozen dependencies which made upgrading dependencies much harder without releasing a new bug fix release of Poetry. This will eventually make our release process easier.
• It relies on standards for the install locations of both Poetry and the poetry script.
• It supports installing from a git repository or a file/directory.
• It no longer tries to modify the PATH but instead will just provide guidance on how to do it if necessary.
• It will use the correct Python interpreter, i.e. the one used to invoke the script, when calling the poetry script.

As to what the script actually does:

• Downloads the virtualenv package to a temporary directory and add it to sys.path.
• Creates a virtual environment in the correct OS data dir which will be
  • %APPDATA%\\pypoetry on Windows
  • ~/Library/Application Support/pypoetry on MacOS
  • ${XDG_DATA_HOME}/pypoetry (or ~/.local/share/pypoetry if it's not set) on UNIX systems
  • In ${POETRY_HOME} if it's set.
• Installs the latest or given version of Poetry inside this virtual environment.
• Installs a poetry script in the Python user directory (or ${POETRY_HOME/bin} if POETRY_HOME is set).

Resolves: #3534
Resolves: #2030
Resolves: #3819
Resolves: #3828
Resolves: #3187

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

[abn]

Over all looks great. Couple of suggestions.

One comment on the virtual environment creation options to ensure some resilience for the installation. Otherwise good to go.

[abn]

Not a blocker, but should we link to our documentation here instead of maintaining duplication information here? Might be good for the brevity of the README as well.

[naveen521kk]

I tried this script and it failed

C:\Users\User> (Invoke-WebRequest -Uri https://raw.githubusercontent.com/python-poetry/poetry/8a506eceb90e7b7d1bc1cd837d303c32e8b03bec/install-poetry.py
-UseBasicParsing).Content | python -
Retrieving Poetry metadata

# Welcome to Poetry!

This will download and install the latest version of Poetry,
a dependency and package manager for Python.

It will add the `poetry` command to Poetry's bin directory, located at:

C:\Users\User\AppData\Roaming\Python\Scripts

You can uninstall at any time by executing this script with the --uninstall opti
on,
and these changes will be reverted.

Installing Poetry (1.1.5)
Installing Poetry (1.1.5): Creating environment
Traceback (most recent call last):
  File "<stdin>", line 808, in <module>
  File "<stdin>", line 804, in main
  File "<stdin>", line 417, in run
  File "<stdin>", line 439, in install
  File "<stdin>", line 503, in make_env
ModuleNotFoundError: No module named 'virtualenv'

It's a fresh install of Python 3.8 if that matters, it doesn't even have pip installed.

EDIT: installing pip fixed that.

[abn]

Spun out the improvement regarding virtualenv bootstrapping to https://github.com/python-poetry/poetry/issues/3843

Otherwise looks good to me.

[abn]

@sdispater this should be good to go after a rebase and conflict resolution.

[github-actions]

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