Fix content assist and type inferencing for extension methods

[eric-milles]

Consider the following:

void m(InputStream s) {
  s.c
}

Content assist for "c" will propose "closeQuietly()" from DefaultGroovyMethodsSupport but this is not an available extension method. If completed, it will be colored as a groovy method and navigation takes you to the declaration. However at runtime there is a MissingMethodException and is STC is enabled an error is shown.