Support importing external packages

[eigengrau]

It would be convenient if it were possible to also import packages not part of the standard library, e.g., via…

#+begin_src go :imports "golang.org/x/exp/constraints@latest"
…
#+end_src

[uakotaobi]

I have a workaround, but it's kind of ugly.

Since Go 1.17, you can't install a package "globally" using go get example.org/foo.

• You can install such a package locally if you're in a directory that has (or whose ancestors have) a go.mod file created with go mod init, or
• You can install a package globally with go install example.org/foo@SOMEVER if and only if it is an executable command. But most Go packages are not stand-alone commands.

Unfortunately, ob-go creates a new temp directory for each of the source blocks it executes, so for most third-party imports, neither of the above points applies.

I got it to work by doing this:

1. I created a known, static location for all ob-go code to run in:

   mkdir $GOPATH/src/babel    
   

2. I turned this directory into a module:

   cd $GOPATH/src/babel
   go mod init github.com/uakotaobi/babel
   

3. I installed third-party packages my ob-go blocks needed by hand:

   go get example.org/foo
   

4. I forced any source blocks requiring third-party code to use that module directory:

   #+name: ob-go-sample-source-block
   #+begin_src go :dir (substitute-in-file-name "$GOPATH/src/babel")
     import (
             "fmt"
             "example.org/foo"
     )
     func main() {
             fmt.Printf("Hello, world\n")
     }
   #+end_src
   

   Steps 1-3 only have to be done once, and step 4 is not needed if a source block is only using builtins.

This is pretty ugly, but I've tested it and it works. It would be better if ob-go automatically turned its temp folders into modules and imported the :imports dependencies with go get.