With the change https://go-review.googlesource.com/#/c/18438/ (per issue #13703), package unsafe is now not special anymore and can be installed and imported like any other package.

Only if it is not found elsewhere is the compiler-known package "unsafe" used.

Make the necessary changes to go/types.

(x/tools/go/types always handled unsafe like other packages - the change is only needed for std lib go/types which handles unsafe directly w/o calling the importer to simplify the importer).