Refactor repository layout and convert build system to Mage.

This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.

tools/vendor/github.com/opennota/check/cmd/varcheck/varcheck.go

@@ -24,13 +24,15 @@ import (
 	"sort"
 	"strings"
 
+	"go/types"
+
 	"github.com/kisielk/gotool"
 	"golang.org/x/tools/go/loader"
-	"go/types"
 )
 
 var (
 	reportExported = flag.Bool("e", false, "Report exported variables and constants")
+	buildTags      = flag.String("tags", "", "Build tags")
 )
 
 type object struct {
@@ -102,6 +104,9 @@ func (v *visitor) Visit(node ast.Node) ast.Visitor {
 		for _, val := range node.Values {
 			ast.Walk(v, val)
 		}
+		if node.Type != nil {
+			ast.Walk(v, node.Type)
+		}
 		return nil
 
 	case *ast.FuncDecl:
@@ -133,6 +138,9 @@ func main() {
 	}
 
 	ctx := build.Default
+	if *buildTags != "" {
+		ctx.BuildTags = strings.Fields(*buildTags)
+	}
 	loadcfg := loader.Config{
 		Build: &ctx,
 	}