gohcl: Composition of types is not supported

Issue description

When using composition of a Go structures, the HCL cannot be parsed using gohcl.

Steps to reproduce the issue

Sample code file:
```
package main
```

import ( "encoding/json" "fmt"

"github.com/hashicorp/hcl2/gohcl"
"github.com/hashicorp/hcl2/hclparse"

)

type File struct { A A hcl:"a,block" json:"a" }

type A struct { B }

type B struct { Foo string json:"foo" hcl:"foo" }

var hclBody = []byte( a { foo = "bar" } )

var jsonBody = []byte( { "a":{ "foo":"bar" } })

func main() { // JSON var j File if err := json.Unmarshal(jsonBody, &j); err != nil { fmt.Println("json.Unmarshal:", err) return } js, _ := json.MarshalIndent(j, "", "\t") fmt.Println(string(js))

// HCL
p := hclparse.NewParser()
f, err := p.ParseHCL(hclBody, "test.hcl")
if err != nil {
    fmt.Println("p.ParseHCL:", err)
    return
}

var h File
if err := gohcl.DecodeBody(f.Body, nil, &h); err != nil {
    fmt.Println("gohcl.DecodeBody:", err)
    return
}
hs, _ := json.MarshalIndent(h, "", "\t")
fmt.Println(string(hs))

}

2. Run the file to see the given error
3. Move the `Foo string` from `B` into `A`
4. Run the file again to see the file working

### What's the expected result

The file is correctly parsed as if `Foo string` from `B` was in `A`, just like its the case with JSON

Output after doing step `3`:
```json
{
    "a": {
        "foo": "bar"
    }
}
{
    "a": {
        "foo": "bar"
    }
}

What's the actual result

Error is being thrown

{
    "a": {
        "foo": "bar"
    }
}
gohcl.DecodeBody: test.hcl:3,2-5: Unsupported argument; An argument named "foo" is not expected here.

Additional details

Go Version: go version go1.12.5 darwin/amd64
Ran go get -u just before running the command, hcl2 at version v0.0.0-20190515223218-4b22149b7cef

hashicorp / hcl