concretizer: first working version with pyclingo interface

- [x] Solver now uses the Python interface to clingo
- [x] can extract unsatisfiable cores from problems when things go wrong
- [x] use Python callbacks for versions instead of choice rules (this may
      ultimately hurt performance)

share/spack/spack-completion.bash

@@ -320,7 +320,7 @@ _spack() {
     then
         SPACK_COMPREPLY="-h --help -H --all-help --color -C --config-scope -d --debug --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
     else
-        SPACK_COMPREPLY="activate add arch blame build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop docs edit env extensions external fetch find flake8 gc gpg graph help info install license list load location log-parse maintainers mirror module patch pkg providers pydoc python reindex remove rm repo resource restage setup spec stage test tutorial undevelop uninstall unload url verify versions view"
+        SPACK_COMPREPLY="activate add arch blame build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop docs edit env extensions external fetch find flake8 gc gpg graph help info install license list load location log-parse maintainers mirror module patch pkg providers pydoc python reindex remove rm repo resource restage setup solve spec stage test tutorial undevelop uninstall unload url verify versions view"
     fi
 }
 
@@ -1464,6 +1464,15 @@ _spack_setup() {
     fi
 }
 
+_spack_solve() {
+    if $list_options
+    then
+        SPACK_COMPREPLY="-h --help --show --models -l --long -L --very-long -I --install-status -y --yaml -j --json -c --cover -N --namespaces -t --types --timers --stats"
+    else
+        _all_packages
+    fi
+}
+
 _spack_spec() {
     if $list_options
     then