Port the existing export command to a plugin

.flake8

@@ -0,0 +1,19 @@
+[flake8]
+max-line-length = 88
+ignore = E501, E203, W503
+per-file-ignores =
+    __init__.py:F401
+exclude =
+    .git
+    __pycache__
+    setup.py
+    build
+    dist
+    releases
+    .venv
+    .tox
+    .mypy_cache
+    .pytest_cache
+    .vscode
+    .github
+    tests/fixtures/

.gitignore

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+*.pyc
+
+# Packages
+*.egg
+!/tests/**/*.egg
+/*.egg-info
+/dist/*
+build
+_build
+.cache
+*.so
+
+# Installer logs
+pip-log.txt
+
+# Unit test / coverage reports
+.coverage
+.tox
+.pytest_cache
+
+.DS_Store
+.idea/*
+.python-version
+.vscode/*
+
+/test.py
+/test_*.*
+
+/setup.cfg
+MANIFEST.in
+/setup.py
+/docs/site/*
+/tests/fixtures/simple_project/setup.py
+.mypy_cache
+
+.venv
+/releases/*
+pip-wheel-metadata
+/poetry.toml

.pre-commit-config.yaml

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+repos:
+  - repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+      - id: black
+
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.8.4
+    hooks:
+      - id: flake8
+
+  - repo: https://github.com/timothycrosley/isort
+    rev: 5.7.0
+    hooks:
+      - id: isort
+        additional_dependencies: [toml]
+        exclude: ^.*/?setup\.py$
+
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v3.4.0
+    hooks:
+      - id: trailing-whitespace
+        exclude: |
+          (?x)(
+            ^tests/.*/fixtures/.*
+          )
+      - id: end-of-file-fixer
+        exclude: ^tests/.*/fixtures/.*
+      - id: debug-statements

README.md

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+# Poetry export plugin
+
+This package is a plugin that allows the export of locked packages to various formats.
+
+**Note**: For now, only the `requirements.txt` format is available.
+
+This plugin provides the same features as the existing `export` command of POetry which it will eventually replace.
+
+
+## Installation
+
+The easiest way to install the `export` plugin is via the `plugin add` command of Poetry.
+
+```bash
+poetry plugin add poetry-export-plugin
+```
+
+If you used `pipx` to install Poetry you can add the plugin via the `pipx inject` command.
+
+```bash
+pipx inject poetry poetry-export-plugin
+```
+
+Otherwise, if you used `pip` to install Poetry you can add the plugin packages via the `pip install` command.
+
+```bash
+pip install poetry-export-plugin
+```
+
+
+## Usage
+
+The plugin provides an `export` command to export to the desired format.
+
+```bash
+poetry export -f requirements.txt --output requirements.txt
+```
+
+**Note**: Only the `requirements.txt` format is currently supported.
+
+### Available options
+
+* `--format (-f)`: The format to export to (default: `requirements.txt`). Currently, only `requirements.txt` is supported.
+* `--output (-o)`: The name of the output file.  If omitted, print to standard output.
+* `--dev`: Include development dependencies.
+* `--extras (-E)`: Extra sets of dependencies to include.
+* `--without-hashes`: Exclude hashes from the exported file.
+* `--with-credentials`: Include credentials for extra indices.