imports: use go/packages, support modules

When in module mode (go env GOMOD is nonempty), use go/packages to load
package information. This adds module support, without slowing down
non-module mode. Functionality should be the same in both cases.

Since we intend to use go/packages for everything eventually, the tests
run in three modes: GOPATH, using the old code paths; GOPATH_GoPackages,
using go/packages on a GOPATH layout; and Modules, using go/packages on
a module layout. They do take a little while to run.

The code path for go/packages is a lot more convoluted than it should be,
since it mixes in and out of the older path. I expect we'll clean that
up after deleting the old one.

Change-Id: I5b54bd470e091e6fb429597dfbfd1c8fa03f5b59
Reviewed-on: https://go-review.googlesource.com/c/152317
Run-TryBot: Heschi Kreinick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Ian Cottrell <[email protected]>

Author: heschi