Update desispec.module

[Waelthus]

This patch updates the DESI_SPECTRO_DARK directory to a new one for upcoming observations. Currently, the new directory only contains symlinks to previous files, but new calibs for post-Y3 observations will go to the new directory only.

[Waelthus]

added a symlink pointing to the most up-to-date directory, using that by default

[sbailey]

Good idea to have a "latest" link so that we can update darks for desispec/main without having to re-install the desispec/main module file.

The jura production environment specifically enforces v2209 so it is safe to update this here. I will merge and update the desispec/main environment at NERSC.

[sbailey]

Having just merged this, I'm no longer sure this is what we want to do, because this will also impact future tags, not just main. I don't think it is viable to assume that whenever we make a desispec tag will will remember to temporarily change this back to a tagged version. Rather than immediately undo this, I'm going to continue the conversation on desihub/desimodules#49 to see if we can converge on what the best practices should be.