go/types, types2: use exact unification when comparing interface methods
Irrespective of whether unification is exact or inexact, method
signatures of interfaces must always match exactly: a type never
satisfies/implements an interface if relevant method signatures
are different (i.e., not identical, possibly after substitution).
Fixes #61879.
Change-Id: I20c0aa28ac86e2edec615b40f2269938e4a96938
Reviewed-on: https://go-review.googlesource.com/c/go/+/519435
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