Provide definitions for implicitly-defined script classes

[trustytrojan]

Changes

• Implement go-to-definition for ClassExpressions, ConstructorCallExpressions, and ImportNodes using implicit script classes, where otherwise scripts were required to return an expression (or have its last statement be an expression) and the definition pointed to that expression.
• Fix NullPointerException when requesting references for a ClassExpression of an implicit script class within the same file. This was found when ctrl+clicking on the expression moduleA within the file moduleA.groovy while another file contained the statement import moduleA. See Fix NullPointerException in ReferenceProvider #118
• Add test cases to GroovyServicesDefinitionTests for implicit script class definition requests

Visual testing

Create the file moduleA.groovy. Do not define an explicit script class like class moduleA {}. Instead write a static and an instance method at the top-level of the script:

static def staticFunc() {}
def instanceFunc() {}

In another Groovy script in the same directory as moduleA.groovy, write:

import moduleA
moduleA.staticFunc()
new moduleA().instanceFunc()

Then ctrl+click or use the "Go To Definition" feature of your editor on moduleA within each statement. Your cursor should be placed at the first character of moduleA.groovy.