Flips the pkgReader.enableAlias flag to true when reading unified IR.
This was disabled while resolving #66873. This resolves the TODO to
flip it back to true.
Updates #66873
Updates #68778
Change-Id: Ifd52b0f9510d6bcf151de1c9a18d71ab548c14e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/604099
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>