[dev.link] cmd/link: stop populating lib/unit.Textp

lib.Textp was used for text address assignment and trampoline
insertion. Now that it has been converted to using the loader,
no need to populate lib.Textp.

Port the logic of canonicalizing dupok symbol's package to the
loader.

unit.Textp was used for DWARF generation, which has also been
converted to using the loader.

Change-Id: I22d4dd30a52a29dd5b1b7b795d43a19f6215e4ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/228140


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>