Appengine Go1.11 beta deployment failure - google-app-engine

Question

I am trying my hands on Go, and started with the go1.11 beta.
The first Hello world example worked fine and I was able to view it via the browser.
In my second attempt, I created multiple .go files (main.go, datastore.go).
I am able to run it locally and it works fine.
However when I attempt to deploy, I get the following error (deployment in first example worked just fine):
gcloud app deploy
...
File upload done.
Updating service [default]...failed.
ERROR: (gcloud.app.deploy) Error Response: [9] Cloud build 67d67177-13cd-4e80-980a-ea36a0760678 status: FAILURE.
Build error details: go build: cannot use -o with multiple packages
On the server, the build logs show:
Step #1 - "builder": building app with command '[go build -o /tmp/staging735225713/usr/local/bin/start ./...]', env '[PATH=/go/bin:/usr/local/go/bin:/builder/google-cloud-sdk/bin/:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin HOSTNAME=d0f271c7f5b8 HOME=/builder/home BUILDER_OUTPUT=/builder/outputs DEBIAN_FRONTEND=noninteractive GOROOT=/usr/local/go/ GOPATH=/go GOPATH=/tmp/staging735225713/srv/gopath]': err=exit status 1, out=go build: cannot use -o with multiple packages
Finished Step #1 - "builder"
ERROR
ERROR: build step 1 "gcr.io/gae-runtimes/go111_app_builder:go111_20181212_RC00" failed: exit status 1
What am I doing wrong?
UPDATE:
Tried various things, including subfolder or a 'vendor' folder as suggested, but no avail.
Updating service [default]...failed.
ERROR: (gcloud.app.deploy) Error Response: [9] Cloud build 79c39e82-3185-4ae1-a7ff-6afa1be448e7 status: FAILURE.
Build error details: srv/proto/myfile.pb.go:8:2: cannot find package "github.com/golang/protobuf/proto" in any of:
/usr/local/go/src/github.com/golang/protobuf/proto (from $GOROOT)
/tmp/staging770905501/srv/gopath/src/github.com/golang/protobuf/proto (from $GOPATH)