spack list: add --tag flag (#32016)

* modified list.py and added functionality for --tag

* Removed long and very long, shifted rest of code above return statement

* removed results variable

* added import statement at top

* added the line accidentally deleted

* added line accidentally deleted

* changed p.name to p, added line inside if statement

* line order switched

* [@spackbot] updating style on behalf of sparkyniner

* ran update completion command

* add tests

* Update lib/spack/spack/test/cmd/list.py

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

* [@spackbot] updating style on behalf of sparkyniner

* changed argument to mock_packages and moved code under filter by tag

* removed bad rebase code and added additional test

* [@spackbot] updating style on behalf of sparkyniner

* added line removed earlier

* added line removed earlier

* replaced function

* added more recommended changes

Co-authored-by: sairaj <sairaj@sairajs-MacBook-Pro.local>
Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

share/spack/spack-completion.bash

@@ -1209,7 +1209,7 @@ _spack_license_update_copyright_year() {
 _spack_list() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -d --search-description --format --update -v --virtuals"
+        SPACK_COMPREPLY="-h --help -d --search-description --format --update -v --virtuals -t --tag"
     else
         _all_packages
     fi