cmd/go: include module root in package index key

cmd/go: include module root in package index key

The package index format includes the directory relative to the module
root. The module root for a given directory can change even if the
contents of the directory itself do not (by adding or removing a
go.mod file in some parent directory).

Thus, we need to invalidate the index for a package when its module
root location changes.

Fixes #53586 (I think).

Change-Id: I2d9f4de80e16bce75b3106a2bad4a11d8378d037
Reviewed-on: https://go-review.googlesource.com/c/go/+/415475
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>