Feature: Allow variants to propagate if not available in source pkg (#42931)

Variants can now be propagated from a dependent package to (transitive) dependencies, 
even if the source or transitive dependencies have the propagated variants.

For example, here `zlib` doesn't have a `guile` variant, but `gmake` does:
```
$ spack spec zlib++guile
 -   zlib@1.3%gcc@12.2.0+optimize+pic+shared build_system=makefile arch=linux-rhel8-broadwell
 -       ^gcc-runtime@12.2.0%gcc@12.2.0 build_system=generic arch=linux-rhel8-broadwell
 -       ^gmake@4.4.1%gcc@12.2.0+guile build_system=generic arch=linux-rhel8-broadwell
```

Adding this property has some strange ramifications for `satisfies()`. In particular:
* The abstract specs `pkg++variant` and `pkg+variant`  do not intersect, because `+variant`
  implies that `pkg` *has* the variant, but `++variant` does not.
* This means that `spec.satisfies("++foo")` is `True` if:
    * for concrete specs: `spec` and its dependencies all have `foo` set if it exists
    * for abstract specs: no dependency of `spec`  has `~foo` (i.e. no dependency contradicts `++foo`).
* This also means that `Spec("++foo").satisfies("+foo")` is `False` -- we only know after concretization.

The `satisfies()` semantics may be surprising, but this is the cost of introducing non-subset
semantics (which are more useful than proper subsets here).

- [x] Change checks for variants
- [x] Resolve conflicts
- [x] Add tests
- [x] Add documentation

---------

Co-authored-by: Gregory Becker <becker33@llnl.gov>
Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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