Before renaming a variable, check the package to make sure that this
renaming would not result in a conflict that could break the program.
All of the implementation is taken from "refactor/rename" with the
dependency on "go/loader" removed.
Change-Id: Ib0782ec8f247a6df1750f2c8213f69186699ce1a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/183257
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
