refactor: taskfile/ast package (#1450)

* refactor: ast package

* feat: read -> taskfile

* refactor: taskfile.Taskfile -> taskfile.Read

* refactor: move merge function back into taskfile package

* refactor: rename taskfile.go to read.go

taskfile/ast/requires.go

@@ -0,0 +1,18 @@
+package ast
+
+import "github.com/go-task/task/v3/internal/deepcopy"
+
+// Requires represents a set of required variables necessary for a task to run
+type Requires struct {
+	Vars []string
+}
+
+func (r *Requires) DeepCopy() *Requires {
+	if r == nil {
+		return nil
+	}
+
+	return &Requires{
+		Vars: deepcopy.Slice(r.Vars),
+	}
+}