move "mage" to a separate module

Move the magefile related files to a submodule, so that it
does not become a dependency for logrus itself.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

ci/magefile.go

@@ -0,0 +1,77 @@
+// +build mage
+
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path"
+
+	"github.com/magefile/mage/mg"
+	"github.com/magefile/mage/sh"
+)
+
+// getBuildMatrix returns the build matrix from the current version of the go compiler
+func getBuildMatrix() (map[string][]string, error) {
+	jsonData, err := sh.Output("go", "tool", "dist", "list", "-json")
+	if err != nil {
+		return nil, err
+	}
+	var data []struct {
+		Goos   string
+		Goarch string
+	}
+	if err := json.Unmarshal([]byte(jsonData), &data); err != nil {
+		return nil, err
+	}
+
+	matrix := map[string][]string{}
+	for _, v := range data {
+		if val, ok := matrix[v.Goos]; ok {
+			matrix[v.Goos] = append(val, v.Goarch)
+		} else {
+			matrix[v.Goos] = []string{v.Goarch}
+		}
+	}
+
+	return matrix, nil
+}
+
+func CrossBuild() error {
+	matrix, err := getBuildMatrix()
+	if err != nil {
+		return err
+	}
+
+	for os, arches := range matrix {
+		for _, arch := range arches {
+			env := map[string]string{
+				"GOOS":   os,
+				"GOARCH": arch,
+			}
+			if mg.Verbose() {
+				fmt.Printf("Building for GOOS=%s GOARCH=%s\n", os, arch)
+			}
+			if err := sh.RunWith(env, "go", "build", "./..."); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func Lint() error {
+	gopath := os.Getenv("GOPATH")
+	if gopath == "" {
+		return fmt.Errorf("cannot retrieve GOPATH")
+	}
+
+	return sh.Run(path.Join(gopath, "bin", "golangci-lint"), "run", "./...")
+}
+
+// Run the test suite
+func Test() error {
+	return sh.RunWith(map[string]string{"GORACE": "halt_on_error=1"},
+		"go", "test", "-race", "-v", "./...")
+}