Improve fbpkg integration

Streamline fbpkg integration to bundle fairseq2 as a packable conda environment. This means: - Using [Feedstock](https://www.internalfb.com/wiki/Metaconda/Building_Conda_envs/Conda_Feedstock/) make package integration more reliable for prod users - Example is [D89733964](https://www.internalfb.com/diff/D89733964), a sandcastle-compatible build script to autodetect and import recent versions of llama4x. Can be done this way for fairseq2