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

0 files changed, 0 insertions, 0 deletions