Add --exclude option to 'spack external find' (#35013)

* Add --exclude option to 'spack external find' to ignore user-specified external packages
* Update bash completion arg order for external find

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

share/spack/spack-completion.bash

@@ -1031,7 +1031,7 @@ _spack_external() {
 _spack_external_find() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --not-buildable -p --path --scope --all -t --tag"
+        SPACK_COMPREPLY="-h --help --not-buildable --exclude -p --path --scope --all -t --tag"
     else
         _all_packages
     fi