cmd/go: handle linking against a shared library that implicitly includes a package

If you say "go install -buildmode=shared a b" and package a depends on another
package c, package c is implicitly included in the resulting shared library (as
specified by "Go Execution Modes"). But if c depends on b, linking against this
shared library hangs, because the go tool doesn't know when computing c's
dependencies that c is part of the same shared library as c.

Fix this by tracking the shared library a package *is* in separately from the
shared library a package has been explicitly linked into.

Fixes #13044

Change-Id: Iacfedab24ae9731ed53d225678b447a2a888823c
Reviewed-on: https://go-review.googlesource.com/16338


Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>