Include cgo files in go#tool#Files listings

The current behaviour (list only pure go files) breaks commands such as go-build for packages mixing both go and cgo.

example:
```
$ go list -f '{{range $f := .GoFiles}}{{$.Dir}}/{{$f}}{{printf "\n"}}{{end}}'
/Users/aam/../dat.go
/Users/aam/../kexttest.go
$ go list -f '{{range $f := .GoFiles}}{{$.Dir}}/{{$f}}{{printf "\n"}}{{end}}{{range $f := .CgoFiles}}{{$.Dir}}/{{$f}}{{printf "\n"}}{{end}}'
/Users/aam/../dat.go
/Users/aam/../kexttest.go
/Users/aam/../kext.go
```