cmd/go: report the imports of CompiledGoFiles in ImportMap

Ideally we should encode the load.PackageInternal data in a way that
doesn't rely on 1:1 correlations of slices, but this is a minimal fix
to unblock Go 1.17.

Fixes #46462

Change-Id: I6e029c69f757aadc54d4be02c01d6b294c217542
Reviewed-on: https://go-review.googlesource.com/c/go/+/326610


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>