How to detect which extras were installed at runtime?

[CAM-Gerlach]

FYI, this was discussed at the PyCon Packaging Summit 2022 and further progress has been made; see:

Provide information on installed extras in package metadata

From discussion at PyCon. Unicorn wants to provide optional features, for example WebSocket support. We do this via an extra, which specifies a minimum supported version of websockets. Unfortunately if users choose to install webscockets without installing our extra, they can install any version and the only way we can decide to enable our disable our websocket support is by trying to import webscockets, parsing the version, etc. One solution to this would be for pip and other tooling to som…