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
66813460c0