Go importer and packages with version flags

[Jack Hill <jackhill@jackhill.us>]

Hi Guix,

While I was was working with the go importer today, it suggested I package 
go-github-com-russross-blackfriday-v2. Fair enough, except we already have 
a package for go-github-com-russross-blackfriday. The packages differ in 
their version (2.1.0 verses 2.0.1), their imputs (the imported package 
definition is missing the inputs and native-inputs that we have in the 
Guix package), and the presense of the v2 versioning disambiguation tag.

I was puzzled by this last bit since both packages seem to compatible with 
the v2 API. If I replace the exising package definition with the one 
suggested by the importer (with one small tweak, I needed to add a /v2 to 
the #:unpack-path) all the packages reported by `guix refresh -l 
go-github-com-russross-blackfriday` build sucessfully.

What's the path forward here? I'm thinking I should update our blackfriday 
backage to the 2.1.0 release, but leave the inputs and native inputs. I'm 
less sure about what to do about the v2 tag. Should I switch over the 
package and varialbe name? What do we usually do?

Best,
Jack

The package the importer suggested:

```
(define-public go-github-com-russross-blackfriday-v2
   (package
     (name "go-github-com-russross-blackfriday-v2")
     (version "2.1.0")
     (source
       (origin
         (method git-fetch)
         (uri (git-reference
                (url "https://github.com/russross/blackfriday")
                (commit (string-append "v" version))))
         (file-name (git-file-name name version))
         (sha256
           (base32 "0d1rg1drrfmabilqjjayklsz5d0n3hkf979sr3wsrw92bfbkivs7"))))
     (build-system go-build-system)
     (arguments
       '(#:import-path
         "github.com/russross/blackfriday/v2"
         #:unpack-path
         "github.com/russross/blackfriday"))
     (home-page "https://github.com/russross/blackfriday")
     (synopsis "Blackfriday")
     (description "Package blackfriday is a markdown processor.
")
     (license license:bsd-2)))
```

Our existing package:

```
(define-public go-github-com-russross-blackfriday
   (package
     (name "go-github-com-russross-blackfriday")
     (version "2.0.1")
     (source
      (origin
        (method git-fetch)
        (uri (git-reference
              (url "https://github.com/russross/blackfriday")
              (commit (string-append "v" version))))
        (file-name (git-file-name name version))
        (sha256
         (base32
          "0nlz7isdd4rgnwzs68499hlwicxz34j2k2a0b8jy0y7ycd2bcr5j"))))
     (build-system go-build-system)
     (arguments
      '(#:import-path "github.com/russross/blackfriday"))
     (propagated-inputs
      `(("go-github-com-shurcool-sanitized-anchor-name"
         ,go-github-com-shurcool-sanitized-anchor-name)))
     (native-inputs
      `(("go-github-com-pmezard-go-difflib" ,go-github-com-pmezard-go-difflib)))
     (home-page "https://github.com/russross/blackfriday")
     (synopsis "Markdown processor in Go")
     (description "Blackfriday is a Markdown processor in Go.")
     (license license:bsd-2)))
```