go-callvis no main packages

1.
Main package is required for the analysis used, because all the calls in the callgraph tree begins in main (and inits).

2.
In package you don't really know what will be called, since it's just a package without some start point. Program that will use the package can call anything or nothing.

tips:
	使用ignore选项来忽略一些依赖：
		go-callvis -ignore os,fmt,strings,io,bufio,regexp,errors,encoding,strconv,time,flag  .