Expose `libpath` in `venv.EnvBuilder.ensure_directories` return value

[pradyunsg]

Feature or enhancement

venv.EnvBuilder.ensure_directories should set libpath as an attribute in the return value.

Pitch

Currently, the value is not exposed in any way via the venv API. This means that API users of venv (ahem... pip) need to recompute this information, coupling strongly to the implementation details of this module.

I noticed this as part of working on pip's build isolation logic, which I'm trying to port to venv.

Previous discussion

This hasn't been discussed extensively yet, but I've posted about this on https://discuss.python.org/t/16796/

[brettcannon]

/cc @vsajip

[brettcannon]

Tangent: @vsajip should we create a expert-venv label?

[vsajip]

Tangent: @vsajip should we create a expert-venv label?

Sure, though I don't know if stuff comes up often enough to merit it.