version: finish version setup

This setup will now correctly set the version number from the git tag.
When using `--version`, we will see the binary name, the package it was
built from and a git hash based on the tag:

```console
$./bin/dist -v
./bin/dist github.com/docker/containerd 0b45d91.m
```

Note that in the above example, if we set a tag of `v1.0.0-dev`, that
will show up in the version number, as follows:

```console
$./bin/dist -v
./bin/dist github.com/docker/containerd v1.0.0-dev
```

Once commits are made past that tag, the version number will be
expressed relative to that tag and include a git hash:

```console
$./bin/dist -v
./bin/dist github.com/docker/containerd v1.0.0-dev-1-g7953e96.m
```

Some these examples include a `.m` postfix. This indicates that the
binary was build from a source tree with local modifications.

We can add a dev tag to start getting 1.0 version numbers for test
builds.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

version.go

@@ -1,20 +1,12 @@
 package containerd
 
-import "fmt"
+var (
+	Package = "github.com/docker/containerd"
 
-// VersionMajor holds the release major number
-const VersionMajor = 1
+	// Version holds the complete version number.
+	Version = "1.0-dev+unknown"
 
-// VersionMinor holds the release minor number
-const VersionMinor = 0
-
-// VersionPatch holds the release patch number
-const VersionPatch = 0
-
-// Version holds the combination of major minor and patch as a string
-// of format Major.Minor.Patch
-var Version = fmt.Sprintf("%d.%d.%d", VersionMajor, VersionMinor, VersionPatch)
-
-// GitCommit is filled with the Git revision being used to build the
-// program at linking time
-var GitCommit = ""
+	// GitCommit is filled with the Git revision being used to build the
+	// program at linking time
+	GitCommit = ""
+)