Fix top level namespace declaration indexing

[vinistock]

Motivation

Closes #2287

The problem surfaced in the issue is actually unrelated to linearizing ancestors. We were incorrectly handling top level namespace declarations and accidentally inserting entries in the index that included a leading ::.

To match Ruby's behaviour, we need to account that a top level reference may be found at any level of nesting and it fully removes all of the preceding namespaces.

Implementation

Created a method to compute the correct nesting. The method searches for the first top level reference in reverse. If a constant with the :: prefix is found, we break out of the loop and return the nesting only up to that point.

Otherwise, we just keep prepending the names into the array and it just results in the same nesting.

Automated Tests

Added two examples that demonstrate the scenarios we did not support.