Improve validation of modules.yaml (#9878)

This PR improves the validation of `modules.yaml` by introducing a custom validator that checks if an attribute listed in `properties` or `patternProperties` is a valid spec. This new check applied to the test case in #9857 gives:

```console
$ spack install szip
==> Error: /home/mculpo/.spack/linux/modules.yaml:5: "^python@2.7@" is an invalid spec [Invalid version specifier]
```

Details: 
* Moved the set-up of a custom validator class to spack.schema
  * In Spack we use `jsonschema` to validate configuration files 
    against a schema. We also need custom validators to enforce
    writing default values within "properties" or "patternProperties"
    attributes.

  * Currently, validators were customized at the place of use and with the
    recent introduction of environments that meant we were setting-up and
    using 2 different validator classes in two different modules.

  * This commit moves the set-up of a custom validator class in the
    `spack.schema` module and refactors the code in `spack.config` and
    `spack.environments` to use it.

* Added a custom validator to check if an attribute is a valid spec
  * Added a custom validator that can be used on objects, which yields an
    error if the attribute is not a valid spec.

* Updated the schema for modules.yaml

* Updated modules.yaml to fix a few inconsistencies:
  - a few attributes were not tested properly using 'anyOf'
  - suffixes has been updated to also check that the attribute is a spec
  - hierarchical_scheme has been updated to hierarchy

* Removed $ref from every schema
  * $ref is not composable or particularly legible
  * Use python dicts and regular old variables instead.