hclwrite: RenameVariablePrefix support index

[magodo]

Currently, the RenameVariablePrefix function has the limitation as mentioned in the comment:

// The search and replacement traversals must be the same length, or this
// method will panic. Only attribute access operations can be matched and
// replaced. Index steps never match the prefix.

This means, following test cases on top of this test will not work:

		{
			Name: "attr traverse of a relative traversal expr sourced by a index expr",
			Config: `resource "foo" "b" {
	count = 3
	name = foo.a[count.index].name
}`,
			OldVar: []string{"foo", "a", "name"},
			NewVar: []string{"foo", "a", "alias"},
			Want: `resource "foo" "b" {
	count = 3
	name = foo.a[count.index].alias
}`,
		},

This is quite a limitation for applications to refactor HCL files for use cases like a language server.

One reason that RenameVariablePrefix not work for above hcl is because the Expression.Variables() in hclsyntax returns partial traversal for cases like the latter case, that the internal walking logic for hclsyntax.RelativeTraversalExpr only tackles with its Source component, but ignore the Traversal component at all.

Whilst the JB HCL plugin somehow could manage this case. So I would like to see the official hclwrite package could also support this.