support github.com/foo/bar/v42

Support vendoring github.com/foo/bar/v42 like the following:

`foo.go`:
```go
package foo

import (
        "github.com/coreos/go-systemd/v22/dbus"
)

func Foo() (*dbus.Conn, error) {
        return dbus.New()
}
```

`vendor.conf`:
```
github.com/coreos/go-systemd/v22 v22.0.0
github.com/godbus/dbus/v5 v5.0.3
```

Now `vndr` checks out the contents of `github.com/coreos/[email protected]` repo
into the `./vendor/github.com/coreos/go-systemd/v22` directory.

Note that `vndr` does not verify the actual version number written in `go.mod`.
So `vndr github.com/coreos/go-systemd/v23 v22.0.0` would vendor v22, not v23.

Fix containerd/cgroups#139

Signed-off-by: Akihiro Suda <[email protected]>

godl/vcs.go

@@ -17,6 +17,7 @@ import (
 	"sync"
 
 	"github.com/LK4D4/vndr/godl/singleflight"
+	"github.com/LK4D4/vndr/versioned"
 )
 
 // envForDir returns a copy of the environment
@@ -512,6 +513,9 @@ func repoRootFromVCSPaths(importPath, scheme string, security securityMode, vcsP
 			repo: match["repo"],
 			root: match["root"],
 		}
+		if versioned.IsVersioned(importPath) {
+			rr.root = importPath
+		}
 		return rr, nil
 	}
 	return nil, errUnknownSite
@@ -581,6 +585,9 @@ func repoRootForImportDynamic(importPath string, security securityMode) (*repoRo
 	if rr.vcs == nil {
 		return nil, fmt.Errorf("%s: unknown vcs %q", urlStr, mmi.VCS)
 	}
+	if versioned.IsVersioned(importPath) {
+		rr.root = importPath
+	}
 	return rr, nil
 }
 

main.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/LK4D4/vndr/build"
 	"github.com/LK4D4/vndr/godl"
+	"github.com/LK4D4/vndr/versioned"
 )
 
 const (
@@ -155,7 +156,7 @@ func validateDeps(deps []depEntry) error {
 		rootDeps := roots[r]
 		if len(rootDeps) == 1 {
 			d := rootDeps[0]
-			if d.importPath != r {
+			if d.importPath != r && !versioned.IsVersioned(d.importPath) {
 				Warnf("package %s is not root import, should be %s", d.importPath, r)
 				invalid = true
 				newDeps = append(newDeps, depEntry{importPath: r, rev: d.rev, repoPath: d.repoPath})

versioned/versioned.go

@@ -0,0 +1,12 @@
+package versioned
+
+import (
+	"regexp"
+)
+
+var re = regexp.MustCompile(`/v[0-9]+$`)
+
+// IsVersioned true for string like "github.com/coreos/go-systemd/v22"
+func IsVersioned(s string) bool {
+	return re.MatchString(s)
+}