Require non-absolute trait method refs to be unambiguous

[nikic]

Currently, when writing something like

class X {
    use T1, T2 {
       func as otherFunc;
    }
    function func() {}
}

where both T1::func() and T2::func() exist, we will simply assume that func refers to T1::func(). This is surprising, and it doesn't really make sense that this particular method gets picked.

This PR validates that non-absolute method references are unambiguous, i.e. refer to exactly one method. If there is ambiguity, it is required to write T1::func as otherFunc or similar.

I think the backwards-compatibility impact here is very low. I did not find any affected code in top 2k packages.

[bishopb]

<3 I'm really surprised that // TODO didn't at least raise a notice.

[nikic]

As there hasn't been any negative feedback on list, I'll go ahead and merge this...