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

.github/workflows/unit_tests.yaml

@@ -130,7 +130,7 @@ jobs:
           sudo apt-get -y update
           # Needed for unit tests
           sudo apt-get -y install \
-              coreutils cvs gfortran graphviz gnupg2 mercurial  ninja-build \
+              coreutils cvs gfortran graphviz gnupg2 mercurial ninja-build \
               patchelf
           # Needed for kcov
           sudo apt-get -y install cmake binutils-dev libcurl4-openssl-dev
@@ -138,6 +138,11 @@ jobs:
     - name: Install Python packages
       run: |
           pip install --upgrade pip six setuptools codecov coverage[toml]
+          # ensure style checks are not skipped in unit tests for python >= 3.6
+          # note that true/false (i.e., 1/0) are opposite in conditions in python and bash
+          if python -c 'import sys; sys.exit(not sys.version_info >= (3, 6))'; then
+              pip install --upgrade flake8 isort>=4.3.5 mypy>=0.900 black
+          fi
     - name: Setup git configuration
       run: |
           # Need this for the git tests to succeed.