Allow specifying values for environment markers during install

[ronaldoussoren]

What's the problem this feature will solve?
Pip currently allows you to specify some, but not all values for environment markers during install (for example the --platform and --implementation options). PEP 496 specifies a number of environment markers that cannot be specified at the moment (in particular os_name and platform_release).

Describe the solution you'd like

Add options --platform-release VALUE and --os-name VALUE to specify the value of platform_release and os_name during the evaluation of platform markers.

My reason for filing the issue is this PyObjC issue: ronaldoussoren/pyobjc#354

This asks for a way to install all sub packages of PyObjC using the pyobjc umbrella package, which uses platform_release to select sub packages that are relevant for the current machine. IMHO the current place to enable this is in pip, not in PyObjC.

Alternative Solutions
I've spent little time to thing about this, but the current workaround for my users is to specify all sub packages manually.

Additional context
None

[uranusjr]

Technically --platform and --implementation are for selecting wheels, not populating PEP 496 environment markers, so this is an entirely different . But this does sound like a reasonable feature request to me.

Mapping markers directly into options feels wrong to me since there are much too many (and may grow in the future). I wonder whether it would be better to come up with some mini-syntax for this, e.g.

pip install --marker="os_name='nt'" --marker="platform_release='...'"

This would avoid the dash-to-underscore conversion as well (which also annoys me slightly).

[aivarannamaa]

Pip currently allows you to specify some, but not all values for environment markers during install (for example the --platform and --implementation options).

According to my experiments, --implementation is ignored when filtering dependencies.

I would also like the possibility to override environment markers, so that https://github.com/aivarannamaa/minipip/ could be more flexible in collecting content for installation to MicroPython and CircuitPython devices. The --marker option looks like a good solution.

[notatallshaw]

This is now being tracked via #11664 ?

[sbidoul]

This is now being tracked via #11664 ?

I think so, yes.