spack external find: change default behavior (#29031)

See https://github.com/spack/spack/issues/25353#issuecomment-1041868116

This commit changes the default behavior of
```
$ spack external find
```
from searching all the possible packages Spack knows about to
search only for the ones tagged as being a "build-tool".

It also introduces a `--all` option to restore the old behavior.

share/spack/spack-completion.bash

@@ -1024,7 +1024,7 @@ _spack_external() {
 _spack_external_find() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --not-buildable --scope -t --tag"
+        SPACK_COMPREPLY="-h --help --not-buildable --scope --all -t --tag"
     else
         _all_packages
     fi