"spack checksum" QoL (#14311)

* Non-interactive mode for spack checksum; allow passing 'package@version' to spack checksum

* Flake8 fixes

* Update checksum.py

Fix typo

* Update spack-completion script

* Automatically set non-interactive mode if more than one version passed

* Update lib/spack/spack/cmd/checksum.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Add documentation and update spack-completion

* Flake8

* Rename option

* Update spack-completion

* Update lib/spack/spack/cmd/checksum.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Update checksum.py

* Update stage.py

* Update create.py

Use batch mode when adding a new package

Co-authored-by: Ivan Razumov <ivan.razumov@cern.ch>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

share/spack/spack-completion.bash

@@ -455,7 +455,7 @@ _spack_cd() {
 _spack_checksum() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --keep-stage"
+        SPACK_COMPREPLY="-h --help --keep-stage -b --batch"
     else
         _all_packages
     fi