[release-branch.r60] reflect: disallow Interface method on Value obtained via unexported name

exp/datafmt is incompatible with this change, so delete it.
It was scheduled for deletion soon anyway.

««« CL 5266054 / f798c2579fbd
reflect: disallow Interface method on Value obtained via unexported name

Had been allowing it for use by fmt, but it is too hard to lock down.
Fix other packages not to depend on it.

R=r, r
CC=golang-dev
https://golang.org/cl/5266054
»»»

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5286051