Fix PackageDescr serialization

Step to reproduce:

1. create a PackageDescrBuilder builder
2. declare types with

   builder.newDeclare().type()...

3. create a PackageDescr descr with

   builder.getDescr()

4. invoke

   descr.writeExternal(ObjectOutput)

The following exception is thrown

java.io.NotSerializableException: org.drools.compiler.lang.descr.QualifiedName

because inside PackageDescr there is a

TypeDeclarationDescr

(that is serializable) that in turn contains

QualifiedName

(that is not)