Cleanup and fix py-torch concretization

• small cleanup to avoid having to specify the compiler per "spec line"
  • introduce "packages" section in env file
  • also move python version constraint there…
• fix py-torch package (forks into local packages/ folder)
• fix py-torch concretization

@akarmas Hmm, maybe I should have split out the "cleanup" part in a separate PR? Should I?

@emuller never mind. The cleanup part is just in the spack.yaml, so in case of failure i will revert it to the previous commit. Thanks, merging now.

merged

mentioned in commit da10a103

@emuller It failed. Logs attached.
py-torch_error_terminal.txtspack-build-out.txt

Ok, seems to be caused by a build regression in py-torch version 1.10 → pinning down to 1.9.1 for now.

(see https://github.com/pytorch/pytorch/issues/68002)