Cleanup: Introduce "packages" section

Previously, we had compiler and python mentioned as specs and relied
on "together" concretization to pin the dependencies. Now, we provide
these constraints via "packages" section, providing global constraints.
Site-specific things, like "mpi" provider/external libs, or architecture
"target" constraints would also go in there…

spack.yaml

 spack:
   specs:
-    # Base
-    - python@3.8.11 %gcc@10.3.0
     # Notebook
-    #- py-jupyter %gcc@10.3.0
-    - py-ipython %gcc@10.3.0
-    - py-ipykernel %gcc@10.3.0
-    - py-notebook %gcc@10.3.0
+    #- py-jupyter
+    - py-ipython
+    - py-ipykernel
+    - py-notebook
     # Collab utils
-    - clb-nb-utils@0.1.0 %gcc@10.3.0
+    - clb-nb-utils@0.1.0
     # General
-    - py-pip %gcc@10.3.0
-    - py-numpy %gcc@10.3.0
-    - py-scipy %gcc@10.3.0
-    - py-pandas %gcc@10.3.0
-    - py-seaborn %gcc@10.3.0
-    - py-matplotlib %gcc@10.3.0
-    - py-nameparser %gcc@10.3.0
-    #- py-version-query %gcc@10.3.0
+    - py-pip
+    - py-numpy
+    - py-scipy
+    - py-pandas
+    - py-seaborn
+    - py-matplotlib
+    - py-nameparser
+    #- py-version-query
     # EBRAINS simulators
-    - nest@3.2 +python +gsl +mpi %gcc@10.3.0
-    - arbor +python +mpi ^python@3:3.9 %gcc@10.3.0
-    - neuron +mpi %gcc@10.3.0
-    - py-pynn@0.9.6 %gcc@10.3.0
-    - tvb-data %gcc@10.3.0
-    - tvb-library ^py-numba@0.54.0 ^binutils+ld+gold %gcc@10.3.0
-    - tvb-storage ^binutils+ld+gold %gcc@10.3.0
-    - py-pyaescrypt %gcc@10.3.0
-    - py-formencode %gcc@10.3.0
-    - tvb-framework ^binutils+ld+gold %gcc@10.3.0
-    - pynn-brainscales@2.0-rc1 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock %gcc@10.3.0
-    #- py-cerebstats %gcc@10.3.0
-    #- py-cerebunit %gcc@10.3.0
-    - py-cerebrus %gcc@10.3.0
-    - py-sciunit@0.2.5.1 %gcc@10.3.0
-    - py-quantities-scidash@0.12.4.3 %gcc@10.3.0
-    #- py-ebrains-drive@0.4.0 %gcc@10.3.0
-    #- py-vf-client@0.6.3 %gcc@10.3.0
-    - py-hbp-archive@1.1.1 %gcc@10.3.0
-    #- py-morphounit@1.0.4 %gcc@10.3.0
-    #- py-neurom@1.4.10 %gcc@10.3.0
-    #- py-morphio@3.1.1 %gcc@10.3.0
-    - py-hippounit@1.3.6 %gcc@10.3.0
-    - py-efel@4.0.4 %gcc@10.3.0
-    - py-hbp-neuromorphic-platform@0.10.1 %gcc@10.3.0
-    #- py-torch~onnx_ml~rocm~tensorpipe~mpi~distributed ^protobuf@:3.17.999 %gcc@10.3.0
-    - py-lfpy@2.2.6 %gcc@10.3.0
-    #- biobb-common %gcc@10.3.0
+    - nest@3.2 +python +gsl +mpi
+    - arbor +python +mpi ^python@3:3.9
+    - neuron +mpi
+    - py-pynn@0.9.6
+    - tvb-data
+    - tvb-library ^py-numba@0.54.0 ^binutils+ld+gold
+    - tvb-storage ^binutils+ld+gold
+    - py-pyaescrypt
+    - py-formencode
+    - tvb-framework ^binutils+ld+gold
+    - pynn-brainscales@2.0-rc1 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock
+    #- py-cerebstats
+    #- py-cerebunit
+    - py-cerebrus
+    - py-sciunit@0.2.5.1
+    - py-quantities-scidash@0.12.4.3
+    #- py-ebrains-drive@0.4.0
+    #- py-vf-client@0.6.3
+    - py-hbp-archive@1.1.1
+    #- py-morphounit@1.0.4
+    #- py-neurom@1.4.10
+    #- py-morphio@3.1.1
+    - py-hippounit@1.3.6
+    - py-efel@4.0.4
+    - py-hbp-neuromorphic-platform@0.10.1
+    #- py-torch~onnx_ml~rocm~tensorpipe~mpi~distributed ^protobuf@:3.17.999
+    - py-lfpy@2.2.6
+    #- biobb-common
     # demo for codejam12
-    #- funniest1022 %gcc@10.3.0
+    #- funniest1022
   concretization: together
+  packages:
+    all:
+      compiler: [gcc@10.3.0]
+    python:
+      version: [3.8.11]