Extract some functionality to its own packages

Like variable and template handling, and logging

internal/compiler/compiler.go

@@ -0,0 +1,12 @@
+package compiler
+
+import (
+	"github.com/go-task/task/internal/taskfile"
+)
+
+// Compiler handles compilation of a task before its execution.
+// E.g. variable merger, template processing, etc.
+type Compiler interface {
+	GetVariables(t *taskfile.Task, call taskfile.Call) (taskfile.Vars, error)
+	HandleDynamicVar(v taskfile.Var) (string, error)
+}