Add Poetry macros to generate Python requirements and constraints

[Eric-Arellano]

Some orgs use Poetry for dependency management because it has an excellent resolver and lockfile support. While they can translate from Poetry into the pip/Pex world of requirements.txt and constraints.txt, this requires running poetry export. Pants can automate some of that via macros, similar to the python_requirements() macro converting requirements.txt into python_requirement_library targets.

We want to distinguish between the high-level pyproject.toml, where the dependencies you want are declared, vs. poetry.lock, where all transitive deps are used. The former should be used to generate python_requirement_library targets, which get used for things like dependency inference. The latter should be used for a constraints file.

Strawman design:

# BUILD
poetry_requirements(
  pyproject="pyproject.toml",   # the default
  lock="poetry.lock",  # the default
  module_mapping={"my_dist": ["module1"]},   # if you want augment Pants's default module mapping
)

Relates to #11710 for better Pipenv support.

[stuhood]

@jsirois : This might relate to some of what you're looking at: would be good to sync with Eric on it when you get back.

[Eric-Arellano]

This is a great feature to have, but not going to make it into 2.0. While this workaround isn't ergonomic, Poetry and Pipenv users can in the meantime use the export commands to export into requirements.txt and constraints.txt files.

We can pick this back up for 2.1 or 2.2, as our focus for the first 2.x releases is futher improving Python, before we venture out to new languages.

[blarghmatey]

Another useful capability if integrating with Poetry and Pipenv is to allow for propagating the constraint ranges to the generated setup.py files when packaging a project. So, rather than setting the install_requires = {'foo==1.2.3'} when Poetry has foo = "^1.0.0", it would generate something like install_requires = {'foo>=1.0.0,<2'}.