support classfile use definition

[luoliwoshang]

The location passed in is when calling Definition in the Rect of hello.gop, but cannot jump to Rect.gox.

//  hello.gop
rect := &Rect{}
println rect

// Rect.gox
var (
	Width, Height int
)
func Area() int {
	return Width * Height
}

Jump position considered.

1. If there is a var, jump to the location of classfile's var.
2. If there is no var, but there is a corresponding function, jump to the location of the first function.
3. Var and function do not exist, jump to the beginning of the file