Changing the package order for the ESM1.6 `spack.yaml`

[manodeep]

Currently the spack.yaml contains the packages in a (seemingly) arbitrary order. Would it be okay to re-order the file into these three groups, and alphabetical order within each group:

1. Top-level exes (mom5, um7, cice5)
2. Intermediate dependencies (oasis3-mct, gcom4, access-fms, access-gtracers, and access-mocsy)
3. System-level dependencies (openmpi, netcdf-c, netcdf-fortran, hdf5)

[Whyborn]

Are you happy to do this @manodeep? Would be easily resolved, would like it to be able to close this before we freeze

[manodeep]

@Whyborn Do you reckon we should do this after code freeze?

Either way, should just be the reordering of the packages and no other changes

[Whyborn]

I don't see why we wouldn't do it before- leave the issues remaining to be only optimisation related.