Add syntactic sugar for "strong preferences" and "conflicts" (#41832)

Currently requirements allow to express "strong preferences" and "conflicts" from
configuration using a convoluted syntax:
```yaml
packages:
  zlib-ng:
    require:
    # conflict on %clang
    - one_of: ["%clang", "@:"]
    # Strong preference for +shared
    - any_of: ["+shared", "@:"]
```
This PR adds syntactic sugar so that the same can be written as:
```yaml
packages:
  zlib-ng:
    conflict:
    - "%clang"
    prefer:
    - "+shared"
```
Preferences written in this way are "stronger" that the ones documented at:
- https://spack.readthedocs.io/en/latest/packages_yaml.html#package-preferences

share/spack/gitlab/cloud_pipelines/stacks/e4s-cray-rhel/spack.yaml

@@ -10,8 +10,8 @@ spack:
 
   packages:
     all:
-      require:
-      - any_of: ["%cce", "@:"]  # cce as a strong preference; not all packages support it
+      prefer:
+      - "%cce"
       compiler: [cce]
       providers:
         blas: [cray-libsci]

share/spack/gitlab/cloud_pipelines/stacks/e4s-oneapi/spack.yaml

@@ -8,8 +8,9 @@ spack:
   packages:
     all:
       require:
-      - any_of: ["%oneapi", "@:"]  # oneapi as a strong preference; not all packages support it
       - "target=x86_64_v3"
+      prefer:
+      - "%oneapi"
       providers:
         blas: [openblas]
         mpi: [mpich]