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/call.go

@@ -0,0 +1,9 @@
+package ast
+
+// Call is the parameters to a task call
+type Call struct {
+	Task   string
+	Vars   *Vars
+	Silent bool
+	Direct bool // Was the task called directly or via another task?
+}