Validate ietf-restconf:yang-data substatements are unique

The template name used by ietf-restconf:yang-data is supposed to be unique within an EffectiveModuleStatement, i.e. this is invalid:

module foo {
  import ietf-restconf { prefix rc; }

  rc:yang-data some {
    container foo;
  }

  rc:yang-data some {
    container bar;
  }
}

We should be rejecting this construct due to template name conflict. Make sure YangDataStatementSupport maintains a dedicated ParserNamespace<String, StmtCtx> keyed by the statement argument. This should be backed by NamespaceBehaviour.statementLocal() stored at the statement's parent.

Also make sure module/submodule conflicts are detected properly.