spack python: add --path option (#22006)

This adds a `--path` option to `spack python` that shows the `python`
interpreter that Spack is using.

e.g.:

```console
$ spack python --path
/Users/gamblin2/src/spack/var/spack/environments/default/.spack-env/view/bin/python
```

This is useful for debugging, and we can ask users to run it to
understand what python Spack is picking up via preferences in `bin/spack`
and via the `SPACK_PYTHON` environment variable introduced in #21222.

share/spack/spack-completion.bash

@@ -1385,7 +1385,7 @@ _spack_pydoc() {
 _spack_python() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -V --version -c -i -m"
+        SPACK_COMPREPLY="-h --help -V --version -c -i -m --path"
     else
         SPACK_COMPREPLY=""
     fi