Extract constant_name method

[andyw8]

This came up while pairing on #3031

@vinistock you initially suggested extracting to GlobalState but it's also used in ReferenceFinder, so I think having on Index makes more sense?

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[andyw8]

@vinistock I've removed the methods that were solely for delegating, but I had intentionally done it that way as I think it improves readability, since there is less noise around those calls.

[vinistock]

Ah, actually, let's keep only the one in common.rb. Otherwise, this is a breaking change as the Rails add-on depends on constant_name.

But let's leave it just to avoid the breaking change, we can use Index.constant_name directly in our implementation.