Support nested closure completions

[gayanper]

Today the plugin only sees the root level extensions and their methods, But if such method supports closure which contains a type, completions are not supported for such closures.

This change tries to find such methods and expand closure collection based on those closure types which could be for example a extension.

Good example of a code snippet to tryout this change is spotless plugin configuration as below

spotless {
    json {
    }
    java {
    }
}

if you try to invoke completions inside json closure, you don't get any and if you try to do the same inside java you endup with config values for standard java plugin.

When this change is applied, the correct configuration values are shown from the relevant spotless extension.

Note: method chaining is still not working which needs to be separately add support.

[gayanper]

@jdneo @testforstephen Let me know what you think