use usersite for getting currently installed packages

[airportyh]

Why?

There's a bug in our Python module setup where removing a package using poetry will remove it from pyproject.toml but not .pythonlibs. It happens because while .pythonlibs is listed as a user site package directory and is considered a standard lib directory, it is not searched as a lib directory for gathering the list of already installed packages.

There's a secondary problem: a bug in poetry where for a "GenericEnv", getting the set of paths never includes the user site package directory because the shell out command to python includes the -I which means isolate and implies
-s : don't add user site directory to sys.path; also PYTHONNOUSERSITE.

I'd like to upstream this change in the future, but that would require a larger change (passing a config parameter to some functions and adding them to various call sites.

Changes

1. Added the user site directory to the paths searched for installed packages if POETRY_USE_USER_SITE is on
2. Use the POETRY_USE_USER_SITE flag for the other place too, that way we can opt-in to this feature
3. Disable the -I flag for when getting paths for the generic environment

[cdmistman]

a bug in poetry where for a "GenericEnv", getting the set of paths never includes the user site package directory because the shell out command to python includes the -I which means isolate and implies
-s : don't add user site directory to sys.path; also PYTHONNOUSERSITE.

I'd like to upstream this change in the future, but that would require a larger change (passing a config parameter to some functions and adding them to various call sites.

Can we at least file the bug upstream and link here? maybe somebody else will pick up the issue, and you can point them to this PR for inspiration

[airportyh]

a bug in poetry where for a "GenericEnv", getting the set of paths never includes the user site package directory because the shell out command to python includes the -I which means isolate and implies
-s : don't add user site directory to sys.path; also PYTHONNOUSERSITE.
I'd like to upstream this change in the future, but that would require a larger change (passing a config parameter to some functions and adding them to various call sites.

Can we at least file the bug upstream and link here? maybe somebody else will pick up the issue, and you can point them to this PR for inspiration

It might be hard to step a reproducible test case for them since they don't have the other 2 changes in this PR. I plan to try to upstream it to their 1.6 alpha properly soon. Just want to get some prod-level testing in first.