Require non-absolute trait method refs to be unambiguous

Currently, when writing something like

class X {
use T1, T2 {
func as otherFunc;
}
functi

Require non-absolute trait method refs to be unambiguous

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 commit 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.

Closes GH-5232.

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