[release-branch.go1.13] cmd/go: fix link error for -coverpkg in GOPATH mode

If a generated test main package transitively depends on a main
package, the main package will now always be rebuilt as a library and
will not be compiled with '-p main'.

This expands the fix for #30907, which only applied to packages with
the BuildInfo set (main packages built in module mode). Linking
multiple packages with BuildInfo caused link errors, but it appears
these errors apply to some symbols in GOPATH mode.

Fixes #34223

Change-Id: Ic1e53437942269a950dd7e45d163707922c92edd
Reviewed-on: https://go-review.googlesource.com/c/go/+/195279


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 24781a1f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/195281