Manually implement merging of Taskfiles and remove dependency on github.com/imdario/mergo

I was carreful enough to check the behavior keeps the same

internal/taskfile/merge.go

@@ -0,0 +1,27 @@
+package taskfile
+
+import (
+	"fmt"
+)
+
+// Merge merges the second Taskfile into the first
+func Merge(t1, t2 *Taskfile) error {
+	if t1.Version != t2.Version {
+		return fmt.Errorf(`Taskfiles versions should match. First is "%s" but second is "%s"`, t1.Version, t2.Version)
+	}
+
+	if t2.Expansions != 0 && t2.Expansions != 2 {
+		t1.Expansions = t2.Expansions
+	}
+	if t2.Output != "" {
+		t1.Output = t2.Output
+	}
+	for k, v := range t2.Vars {
+		t1.Vars[k] = v
+	}
+	for k, v := range t2.Tasks {
+		t1.Tasks[k] = v
+	}
+
+	return nil
+}