Arbitrary Equality Requirements

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• I have searched the FAQ and general documentation and believe that my question is not already covered.

Feature Request

Hi, I think poetry should support arbitrary equality requirements as described in PEP 440. Current behavior is problematic in case of the following usage:

[tool.poetry.dependencies]
xgboost= [
    {version = "0.81+upstream", markers = "sys_platform == 'linux'", source="***"},
    {version = "0.81", markers = "sys_platform != 'linux'", source="***"},
]

Results in only the 0.81+upstream version ever being installed (as though installed with pip install xgboost==0.81, which too installs 0.81+upstream if available)

I propose to make the following requirement legal

[tool.poetry.dependencies]
xgboost= [
    {version = "0.81+upstream", markers = "sys_platform == 'linux'", source="***"},
    {version = "===0.81", markers = "sys_platform != 'linux'", source="***"},
]

Which will install 0.81+upstream on linux systems and 0.81 on non-linux systems (install as though with pip install xgboost===0.81)

The issue names no source files, tests, or entry points. Start by reading PEP 440's arbitrary-equality section and reproducing the xgboost dependency examples; trace Poetry's requirement parsing and resolution behavior, with done meaning ===0.81 is accepted and selects the requested versions under the stated platform markers.