cmd/go: force -coverpkg main packages to be built as libraries

This fixes TestScript/cover_pkgall_multiple_mains, which started
failing after CL 174657.

When compiling main packages with coverage instrumentation
(e.g., for -coverpkg all), we now pass -p with the full import path
instead of '-p main'. This avoids link errors
'duplicate symbol main.main (types 1 and 1)'.

Fixes #31946

Change-Id: Id147527b1dbdc14bb33ac133c30d50c250b4365c
Reviewed-on: https://go-review.googlesource.com/c/go/+/176558


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>