concretizer args: --fresh-roots == --reuse-deps (#43988)

Since reuse is the default now, `--reuse-deps` can be confusing, as it
technically does not imply roots are fresh.

So add `--fresh-roots`, which is also easier to discover when running
`spack concretize --fre<tab>`

lib/spack/spack/cmd/common/arguments.py

@@ -563,12 +563,13 @@ def add_concretizer_args(subparser):
         help="reuse installed packages/buildcaches when possible",
     )
     subgroup.add_argument(
+        "--fresh-roots",
         "--reuse-deps",
         action=ConfigSetAction,
         dest="concretizer:reuse",
         const="dependencies",
         default=None,
-        help="reuse installed dependencies only",
+        help="concretize with fresh roots and reused dependencies",
     )
     subgroup.add_argument(
         "--deprecated",

lib/spack/spack/test/cmd/common/arguments.py

@@ -123,7 +123,13 @@ def test_root_and_dep_match_returns_root(mock_packages, mutable_mock_env_path):
 
 
 @pytest.mark.parametrize(
-    "arg,conf", [("--reuse", True), ("--fresh", False), ("--reuse-deps", "dependencies")]
+    "arg,conf",
+    [
+        ("--reuse", True),
+        ("--fresh", False),
+        ("--reuse-deps", "dependencies"),
+        ("--fresh-roots", "dependencies"),
+    ],
 )
 def test_concretizer_arguments(mutable_config, mock_packages, arg, conf):
     """Ensure that ConfigSetAction is doing the right thing."""