samples: move go.mod into samples root, making samples its own module (#2057)

Updates #2048 - moves the go.mod file from samples/appengine to samples.

Note the replace gocloud.dev => ../ added to the go.mod. Without this, running go test ./... in samples/ results in many errors like:

can't load package: package gocloud.dev/samples/server: unknown import path "gocloud.dev/samples/server": ambiguous import: found gocloud.dev/samples/server in multiple modules:
	gocloud.dev/samples (/home/eliben/eli/go-cloud/samples/server)
	gocloud.dev v0.13.0 (/home/eliben/eli/go/pkg/mod/[email protected]/samples/server)

The symptom and solution is explained by bcmills in ugorji/go#279 (which refers also to golang/go#27899). The new go.mod points to gocloud.dev v0.13, which also provides these packages - so the go command is confused - it sees the same package(s) provided by two different modules.

The ugorji/go solution was to use a pseudo-version pointing at an existing commit in the core module which removes the packages - this removes the ambiguity. In our case, there is no existing commit yet - so I'm using a replace line. The replace line should be unnecessary when we release a new CDK version.

This has interesting implications for #886 - we'll likely have to do the same when we split out providers to their own modules and retain replace lines until a new release.

Author: eliben

internal/testing/alldeps

@@ -32,7 +32,6 @@ github.com/google/go-cmp
 github.com/google/go-github
 github.com/google/go-querystring
 github.com/google/martian
-github.com/google/subcommands
 github.com/google/uuid
 github.com/google/wire
 github.com/googleapis/gax-go
@@ -77,7 +76,7 @@ gocloud.dev
 gocloud.dev/internal/cmd/gocdk
 gocloud.dev/internal/contributebot
 gocloud.dev/internal/website
-gocloud.dev/samples/appengine/helloworld
+gocloud.dev/samples
 golang.org/x/crypto
 golang.org/x/net
 golang.org/x/oauth2
@@ -91,6 +90,5 @@ google.golang.org/api
 google.golang.org/appengine
 google.golang.org/genproto
 google.golang.org/grpc
-gopkg.in/pipe.v2
 gopkg.in/square/go-jose.v2
 pack.ag/amqp

samples/appengine/go.mod

@@ -0,0 +1,19 @@
+module gocloud.dev/samples
+
+require (
+	contrib.go.opencensus.io/exporter/stackdriver v0.11.0
+	git.apache.org/thrift.git v0.12.0 // indirect
+	github.com/Azure/azure-pipeline-go v0.1.9
+	github.com/Azure/azure-storage-blob-go v0.6.0
+	github.com/aws/aws-sdk-go v1.19.16
+	github.com/go-sql-driver/mysql v1.4.1
+	github.com/google/subcommands v1.0.1
+	github.com/google/wire v0.2.1
+	github.com/gorilla/mux v1.7.1
+	go.opencensus.io v0.21.0
+	gocloud.dev v0.13.0
+	google.golang.org/genproto v0.0.0-20190418145605-e7d98fc518a7
+	gopkg.in/pipe.v2 v2.0.0-20140414041502-3c2ca4d52544
+)
+
+replace gocloud.dev => ../