cmd/internal/objabi: add inverse of PathToPrefix

cmd/internal/objabi: add inverse of PathToPrefix

Add PrefixToPath, which can be used to convert a package path in a
symbol name back to the original package path.

For #61577.

Change-Id: Ifbe8c852a7f41ff9b81ad48b92a26a0e1b046777
Reviewed-on: https://go-review.googlesource.com/c/go/+/529557
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>