spack style: add --root option (#25085)

This adds a `--root` option so that `spack style` can check style for
a spack instance other than its own.

We also change the inner workings of `spack style` so that `--config FILE`
(and similar options for the various tools) options are used. This ensures
that when `spack style` runs, it always uses the config from the running spack,
and does *not* pick up configuration from the external root.

- [x] add `--root` option to `spack style`
- [x] add `--config` (or similar) option when invoking style tools
- [x] add a test that verifies we can check an external instance

share/spack/spack-completion.bash

@@ -1008,7 +1008,7 @@ _spack_find() {
 _spack_flake8() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -b --base -a --all -r --root-relative -U --no-untracked -f --fix --no-isort --no-flake8 --no-mypy --black"
+        SPACK_COMPREPLY="-h --help -b --base -a --all -r --root-relative -U --no-untracked -f --fix --no-isort --no-flake8 --no-mypy --black --root"
     else
         SPACK_COMPREPLY=""
     fi
@@ -1614,7 +1614,7 @@ _spack_stage() {
 _spack_style() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -b --base -a --all -r --root-relative -U --no-untracked -f --fix --no-isort --no-flake8 --no-mypy --black"
+        SPACK_COMPREPLY="-h --help -b --base -a --all -r --root-relative -U --no-untracked -f --fix --no-isort --no-flake8 --no-mypy --black --root"
     else
         SPACK_COMPREPLY=""
     fi