runtime/local: add support for idiomatic folder structures (#1963)

* runtime/local: add support for idiomatic folder structures

* runtime/local: add test coverage

* runtime/local: increase test coverage

* runtime/local: add test for empty local source

* runtime/local: make entrypoint public

runtime/local/local_test.go

@@ -0,0 +1,33 @@
+package local
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestEntrypoint(t *testing.T) {
+	wd, _ := os.Getwd()
+
+	// test a service with idiomatic folder structure
+	result, err := Entrypoint(filepath.Join(wd, "test"))
+	assert.Nil(t, err, "Didn'expected entrypoint to return an error")
+	assert.Equal(t, "cmd/test/main.go", result, "Expected entrypoint to return cmd/test/main.go")
+
+	// test a service with a top level main.go
+	result, err = Entrypoint(filepath.Join(wd, "test/bar"))
+	assert.Nil(t, err, "Didn'expected entrypoint to return an error")
+	assert.Equal(t, "main.go", result, "Expected entrypoint to return main.go")
+
+	// test a service with multiple main.go files within the cmd folder
+	result, err = Entrypoint(filepath.Join(wd, "test/foo"))
+	assert.Error(t, err, "Expected entrypoint to return an error when multiple main.go files exist")
+	assert.Equal(t, "", result, "Expected entrypoint to not return a result")
+
+	// test a service with no main.go files
+	result, err = Entrypoint(filepath.Join(wd, "test/empty"))
+	assert.Error(t, err, "Expected entrypoint to return an error when no main.go files exist")
+	assert.Equal(t, "", result, "Expected entrypoint to not return a result")
+}