轉載出處：本文來源
package,source folder,folder 之間相互轉換
package 轉成 folder 顯示：選中package, build path-> Exclude
folder 轉成 package 顯示：選中folder, build path-> Include
package 轉成 source folder 顯示：選中package, build path-> Use as Source folder
source folder 轉成 package 顯示：選中folder, build path-> Remove from BuildPath
pacage 與 source folder 的轉換同上