Instantiate EffectiveRevisionDate statements on linkage

Our parser does not reflect the result of inter-module linkage (via import) into the effective model. This means users trying to cross-reference EffectiveModuleStatements need to perform their own re-linkage because they have no way of finding out how a particular import was resolved.

Improve parser so that for a revisionless import:

module foo {
  import bar {
    prefix bar;
  }
}

we introduce an undeclared EffectiveRevisionDate statement which reflects the revision of the imported module. i.e. when linked with:

module bar {
  revision 2010-11-12;
}

the effective model of foo looks as if it was declared: 

module foo {
  import bar {
    prefix bar;
    revision-date 2010-11-12;
  }
}