Go internal package内部包
Go中命名为internal的package，只有该package的父级package才可以访问该package的内容。
两点需要注意：
- .只有直接父级package可以访问，其他的都不行，再往上的祖先package也不行
- 父级package也只能访问internal package使用大写暴露出的内容，小写的不行
Go官方文档对internal package有相关的描述，粘贴如下：
https://golang.org/doc/go1.4#internalpackages
Go’s package system makes it easy to structure programs into components with clean boundaries, but there are only two forms of access: local (unexported) and global (exported). Sometimes one wishes to have components that are not exported, for instance to avoid acquiring clients of interfaces to code that is part of a public repository but not intended for use outside the program to which it belongs.
The Go language does not have the power to enforce this distinction, but as of Go 1.4 the go command introduces a mechanism to define “internal” packages that may not be imported by packages outside the source subtree in which they reside.
To create such a package, place it in a directory named internal or in a subdirectory of a directory named internal. When the go command sees an import of a package with internal in its path, it verifies that the package doing the import is within the tree rooted at the parent of the internal directory. For example, a package …/a/b/c/internal/d/e/f can be imported only by code in the directory tree rooted at …/a/b/c. It cannot be imported by code in …/a/b/g or in any other repository.
For Go 1.4, the internal package mechanism is enforced for the main Go repository; from 1.5 and onward it will be enforced for any repository.