plugins: correctly output build version, cosmetic cleanups

Now that libcni has the ability to print a version message, plumb it
through correctly.

While we're at it,
- fix import paths
- run gofmt
- add some more comments to sample
- add container runtime swappability for release

pkg/utils/buildversion/buildversion.go

@@ -0,0 +1,26 @@
+// Copyright 2019 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Buildversion is a destination for the linker trickery so we can auto
+// set the build-version
+package buildversion
+
+import "fmt"
+
+// This is overridden in the linker script
+var BuildVersion = "version unknown"
+
+func BuildString(pluginName string) string {
+	return fmt.Sprintf("CNI %s plugin %s", pluginName, BuildVersion)
+}