feat(refacto): move everything in internal package

This allows to install katenary with `go install` and to clean up the
project folder.

internal/utils/hash.go

@@ -0,0 +1,26 @@
+package utils
+
+import (
+	"crypto/sha1"
+	"encoding/hex"
+	"io"
+	"os"
+	"sort"
+)
+
+// HashComposefiles returns a hash of the compose files.
+func HashComposefiles(files []string) (string, error) {
+	sort.Strings(files) // ensure the order is always the same
+	sha := sha1.New()
+	for _, file := range files {
+		f, err := os.Open(file)
+		if err != nil {
+			return "", err
+		}
+		defer f.Close()
+		if _, err := io.Copy(sha, f); err != nil {
+			return "", err
+		}
+	}
+	return hex.EncodeToString(sha.Sum(nil)), nil
+}