Packages that use docbook-xml may specify a specific entity version.
When this is specified as a version constraint in the package recipe it
will cause problems when using `unify = True` in a Spack environment, as
there could be multiple versions of docbook-xml in the spec. In
practice, any entity version should work with any other version and
everything should work with the latest version. This PR maps all Spack
docbook-xml entity versions to the docbook-xml version in the spec.
Ideally, the version in the spec would be the latest version. With this
PR, even if a package specifies an older entity version, it will map
to the entity version (latest) in the spec. This means that there can be one
docbook-xml version in a Spack environment spec and packages requesting
older entity versions will still work.
To help facilitate this, docbook-xml version constraints for packages
that have them have been removed. Those packages are dbus and gtk-doc.
fcf9c639ee