It's important for a smooth transition for non-module users
not to change operation in GOPATH/src by default in Go 1.11,
even if go.mod exists in a downloaded dependency.

Even so, users create go.mod and then are confused about
why 'go get' commands seem to behave oddly, when in fact
they are getting the old 'go get'.

Try to split the difference by printing a warning in 'go get'
when run in a tree that would normally be considered a
module if only it were outside GOPATH/src.

Change-Id: I55a1cbef127f3f36de54a8d7b93e1fc64bf0a708
Reviewed-on: https://go-review.googlesource.com/124859Reviewed-by: Bryan C. Mills <bcmills@google.com>