Add support for variants without custom cores (ie, EnviroDIY Stonefly using Adafruit core)

[SRGDamia1]

I have the JSON and variant files working correctly on my local computer, but I want to make them available in the registry for all users. This board does not need a separate core; it uses the Adafruit core. It does need a variants file, though. I can submit a PR for the board .json file, but the build system only accepts variant files stored locally in the project directory or as part of a complete core hosted elsewhere.

https://github.com/platformio/platform-atmelsam/blob/4415ce442d20067c749324d3953c460d881cb3f7/builder/frameworks/arduino/arduino-samd.py#L180-L191

I want PlatformIO to automatically find and download the variants file for the user without me maintaining a copy of the Adafruit core.

If I'm not understanding and there's already a way to do this, please let me know!