Allow conditional possible values in variants (#29530)

Allow declaring possible values for variants with an associated condition. If the variant takes one of those values, the condition is imposed as a further constraint.

The idea of this PR is to implement part of the mechanisms needed for modeling [packages with multiple build-systems]( https://github.com/spack/seps/pull/3). After this PR the build-system directive can be implemented as:
```python
variant(
    'build-system',
    default='cmake',
    values=(
        'autotools',
        conditional('cmake', when='@X.Y:')
    ), 
    description='...',
)
```

Modifications:
- [x] Allow conditional possible values in variants
- [x] Add a unit-test for the feature
- [x] Add documentation

var/spack/repos/builtin.mock/packages/conditional-values-in-variant/package.py

@@ -0,0 +1,34 @@
+# Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class ConditionalValuesInVariant(Package):
+    """Package with conditional possible values in a variant"""
+    homepage = "https://dev.null"
+
+    version('1.73.0')
+    version('1.72.0')
+    version('1.62.0')
+    version('1.60.0')
+    version('1.50.0')
+
+    variant(
+        'cxxstd', default='98',
+        values=(
+            '98', '11', '14',
+            # C++17 is not supported by Boost < 1.63.0.
+            conditional('17', when='@1.63.0:'),
+            # C++20/2a is not support by Boost < 1.73.0
+            conditional('2a', when='@1.73.0:')
+        ),
+        multi=False,
+        description='Use the specified C++ standard when building.',
+        when='@1.60.0:'
+    )
+
+    variant(
+        'staging', values=any_combination_of(
+            conditional('flexpath', 'dataspaces', when='@1.73.0:')
+        ),
+        description='Enable dataspaces and/or flexpath staging transports'
+    )

var/spack/repos/builtin/packages/boost/package.py

@@ -140,7 +140,13 @@ def libs(self):
 
     variant('cxxstd',
             default='98',
-            values=('98', '11', '14', '17', '2a'),
+            values=(
+                '98', '11', '14',
+                # C++17 is not supported by Boost < 1.63.0.
+                conditional('17', when='@1.63.0:'),
+                # C++20/2a is not support by Boost < 1.73.0
+                conditional('2a', when='@1.73.0:')
+            ),
             multi=False,
             description='Use the specified C++ standard when building.')
     variant('debug', default=False,
@@ -193,12 +199,6 @@ def libs(self):
     conflicts('cxxstd=98', when='+fiber')  # Fiber requires >=C++11.
     conflicts('~context', when='+fiber')  # Fiber requires Context.
 
-    # C++20/2a is not support by Boost < 1.73.0
-    conflicts('cxxstd=2a', when='@:1.72')
-
-    # C++17 is not supported by Boost<1.63.0.
-    conflicts('cxxstd=17', when='@:1.62')
-
     conflicts('+taggedlayout', when='+versionedlayout')
     conflicts('+numpy', when='~python')