DSLD: fix support for script fields and variables

[eric-milles]

Consider the following:
some.dsld

NAME = 'bar'
contribute(currentType('Foo')) {
  property name:NAME, type:Integer
}

Foo.groovy

class Foo {
  void test() {
    bar
  }
}

Property "bar" should infer as an integer. However, it fails because DSLContributionGroup is the delegate for the DSLD's contribution closure and its getProperty(String) intercepts all property requests.