Implement `gopackages` subcommand

[DQNEO]

$ gopackages -mode types ./lib/mylib
Go package "github.com/DQNEO/babygo/lib/mylib":
    package mylib
    has complete exported type info
    file /Users/DQNEO/src/github.com/DQNEO/babygo/lib/mylib/mylib.go
    import "github.com/DQNEO/babygo/lib/mylib2"
    import "os"
    func InArray(x string, list []string) bool
    const Msg string
    func Readdirnames(dir string) ([]string, error)
    func SortStrings(ss []string)
    func Sum(a int, b int) int
    func Sum2(a int, b int) int
    type Type struct{Field int}
    method (*Type) Method() int

TODO

• Implement babygo gopackages <pkg>
• Ensure all packages output the same result
• Add test in Makefile to check the results. (compare babygo 2gen with the official gopackages)