go/build: import packages in module mode when GO111MODULE is "on"

go/build: import packages in module mode when GO111MODULE is "on"

go/build.Import locates package dirctories using 'go list' when in
module mode (finding, downloading, and extracting modules is
complicated, so go/build does not handle it).

Previously, Import used 'go list' if GO111MODULE was not explicitly
off and a go.mod file was present (plus some other conditions). With
this change, if GO111MODULE is "on", a go.mod file does not need to be
present.

Fixes #34669

Change-Id: I9e56871054d4b07c3fc04b6f14a5c8c8e9f3c333
Reviewed-on: https://go-review.googlesource.com/c/go/+/199818
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>