debug: move "nonexistent config path" message to much higher verbosity level (#34201)

We currently report that searched config paths don't exist at debug level 1, which
clutters the output quite a bit:

```console
> spack -d solve --fresh --show asp hdf5 > hdf5.lp
==> [2022-11-29-14:18:21.035133] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/darwin/concretizer.yaml
==> [2022-11-29-14:18:21.035151] Skipping nonexistent config path /Users/gamblin2/.spack/concretizer.yaml
==> [2022-11-29-14:18:21.035169] Skipping nonexistent config path /Users/gamblin2/.spack/darwin/concretizer.yaml
==> [2022-11-29-14:18:21.035238] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/repos.yaml
==> [2022-11-29-14:18:21.035996] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/defaults/darwin/repos.yaml
==> [2022-11-29-14:18:21.036021] Skipping nonexistent config path /etc/spack/repos.yaml
==> [2022-11-29-14:18:21.036039] Skipping nonexistent config path /etc/spack/darwin/repos.yaml
==> [2022-11-29-14:18:21.036057] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/repos.yaml
==> [2022-11-29-14:18:21.036072] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/darwin/repos.yaml
==> [2022-11-29-14:18:21.036088] Skipping nonexistent config path /Users/gamblin2/.spack/repos.yaml
==> [2022-11-29-14:18:21.036105] Skipping nonexistent config path /Users/gamblin2/.spack/darwin/repos.yaml
==> [2022-11-29-14:18:21.071828] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/config.yaml
==> [2022-11-29-14:18:21.081628] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/defaults/darwin/config.yaml
==> [2022-11-29-14:18:21.081669] Skipping nonexistent config path /etc/spack/config.yaml
==> [2022-11-29-14:18:21.081692] Skipping nonexistent config path /etc/spack/darwin/config.yaml
==> [2022-11-29-14:18:21.081712] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/config.yaml
==> [2022-11-29-14:18:21.081731] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/darwin/config.yaml
==> [2022-11-29-14:18:21.081748] Skipping nonexistent config path /Users/gamblin2/.spack/config.yaml
==> [2022-11-29-14:18:21.081764] Skipping nonexistent config path /Users/gamblin2/.spack/darwin/config.yaml
==> [2022-11-29-14:18:21.134909] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/packages.yaml
==> [2022-11-29-14:18:21.148695] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/darwin/packages.yaml
==> [2022-11-29-14:18:21.152555] Skipping nonexistent config path /etc/spack/packages.yaml
==> [2022-11-29-14:18:21.152582] Skipping nonexistent config path /etc/spack/darwin/packages.yaml
==> [2022-11-29-14:18:21.152601] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/packages.yaml
==> [2022-11-29-14:18:21.152620] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/darwin/packages.yaml
==> [2022-11-29-14:18:21.152637] Skipping nonexistent config path /Users/gamblin2/.spack/packages.yaml
==> [2022-11-29-14:18:21.152654] Skipping nonexistent config path /Users/gamblin2/.spack/darwin/packages.yaml
==> [2022-11-29-14:18:21.853915] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/defaults/compilers.yaml
==> [2022-11-29-14:18:21.853962] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/defaults/darwin/compilers.yaml
==> [2022-11-29-14:18:21.853987] Skipping nonexistent config path /etc/spack/compilers.yaml
==> [2022-11-29-14:18:21.854007] Skipping nonexistent config path /etc/spack/darwin/compilers.yaml
==> [2022-11-29-14:18:21.854025] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/compilers.yaml
==> [2022-11-29-14:18:21.854043] Skipping nonexistent config path /Users/gamblin2/src/spack/etc/spack/darwin/compilers.yaml
==> [2022-11-29-14:18:21.854060] Skipping nonexistent config path /Users/gamblin2/.spack/compilers.yaml
==> [2022-11-29-14:18:21.854093] Reading config from file /Users/gamblin2/.spack/darwin/compilers.yaml
```

It is very rare that I want to know this much information about config search, so I've
moved this to level 3. Now at level 1, we can see much more clearly what configs were
actually found:

```console
> spack -d solve --fresh --show asp hdf5 > hdf5.lp
==> [2022-11-29-14:19:04.035457] Imported solve from built-in commands
==> [2022-11-29-14:19:04.035818] Imported solve from built-in commands
==> [2022-11-29-14:19:04.037626] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/concretizer.yaml
==> [2022-11-29-14:19:04.040033] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/repos.yaml
==> [2022-11-29-14:19:04.080852] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/config.yaml
==> [2022-11-29-14:19:04.133241] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/packages.yaml
==> [2022-11-29-14:19:04.147175] Reading config from file /Users/gamblin2/src/spack/etc/spack/defaults/darwin/packages.yaml
==> [2022-11-29-14:19:05.157896] Reading config from file /Users/gamblin2/.spack/darwin/compilers.yaml
```

You can still get the old messages with `spack -ddd` (to activate debug level 3).

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index c3165d1c9c..2ecb500390 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -737,7 +737,7 @@ def override(path_or_scope, value=None):
 
 #: configuration scopes added on the command line
 #: set by ``spack.main.main()``.
-command_line_scopes = []  # type: List[str]
+command_line_scopes: List[str] = []
 
 
 def _add_platform_scope(cfg, scope_type, name, path):
@@ -989,7 +989,7 @@ def read_config_file(filename, schema=None):
 
     if not os.path.exists(filename):
         # Ignore nonexistent files.
-        tty.debug("Skipping nonexistent config path {0}".format(filename))
+        tty.debug("Skipping nonexistent config path {0}".format(filename), level=3)
         return None
 
     elif not os.path.isfile(filename):