BUG: Fix poetry / pdm usage

[HaoZeke]

Closes #1281. Also implements handing poetry and pdm with a much cleaner approach.

[DavidRConnell]

This is working for my poetry project. Thank you. Having pip handle the different backends like this is a nicer solution. Surprised pip doesn't automatically install the backend.

Should the requirement for poetry packages be poetry-core instead of poetry (line 507)? installing poetry-core is working for me as well and the poetry-core README indicates it is meant to replace the full poetry package for this purpose.

[HaoZeke]

This is working for my poetry project. Thank you. Having pip handle the different backends like this is a nicer solution. Surprised pip doesn't automatically install the backend.

Excellent, thanks for confirming and yeah it would make much more sense for pip to respect build-backend...

Should the requirement for poetry packages be poetry-core instead of poetry (line 507)? installing poetry-core is working for me as well and the poetry-core README indicates it is meant to replace the full poetry package for this purpose.

It wasn't immediately clear to me that poetry.core is enough to handle all the additional fields poetry uses, e.g. I thought it would be ok for a pyproject.toml which just builds with poetry but not if there was heavy use of the keys under [tool.poetry]... I think it should be ok though, am updating, thanks for the heads up!

[mattip]

Thanks @HaoZeke