support meta name="go-import" to detect Go repository

[Songmu]

follow on #119

Before

% ghq get golang.org/x/crypto
     clone https://golang.org/x/crypto -> /Users/Songmu/dev/src/golang.org/x/crypto
       git ls-remote https://golang.org/x/crypto
        hg identify https://golang.org/x/crypto
       svn info https://golang.org/x/crypto
     error Could not find version control system: https://golang.org/x/crypto

After

$ ghq get golang.org/x/crypto
     clone https://golang.org/x/crypto -> /Users/Songmu/dev/src/golang.org/x/crypto
       git ls-remote https://golang.org/x/crypto
       git clone https://go.googlesource.com/crypto /Users/Songmu/dev/src/golang.org/x/crypto
Cloning into '/Users/Songmu/dev/src/golang.org/x/crypto'...
remote: Total 5013 (delta 2982), reused 5013 (delta 2982)
Receiving objects: 100% (5013/5013), 4.76 MiB | 7.54 MiB/s, done.
Resolving deltas: 100% (2982/2982), done.

[Songmu]

Since this "meta go-import" is a Go-specific specification, there is a concern about being tightly coupled to it.

However, since go get will no longer clone the source in GOPATH after go1.13, it is inconvenient for those who used ghq to access sources, so I make ghq to be possible to clone it.

I want to merge it because it is useful for Go programmers.