Add support for dependency groups

[sdispater]

This PR adds support for dependency groups at the core level (PR on the Poetry side to follow).

Basically, dependency groups can be declared in the pyproject.toml file like so:

[tool.poetry.group.dev.dependencies]
black = "*"

[tool.poetry.group.test.dependencies]
pytest = "*"

There is also support for optional dependency groups that can be opted in:

[tool.poetry.group]
optional = true

[tool.poetry.group.docsdependencies]
mkdocs = "*"

Groups can be excluded, opted in and exclusively selected via specific methods on the Package class.

Resolves: python-poetry/poetry#1644

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

[sonarqubecloud]

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[davidszotten]

is the plan to keep these groups as a separate concept to tool.poetry.extras (ie to generate extras_require)