Fix type inference of local variables vs. DSLD contributions

Consider the following:

some.dsld
```groovy
contribute(enclosingMethod('foo')) {
  property name: 'bar', type: BigInteger
}
```

Script.groovy
```groovy
def foo(Integer bar) {
  bar // should inference as Integer, not BigInteger
}
```

At present, DSLD contributions (from `method` and `property`) are overriding local parameter/variable declarations.