See golang/lint#61 for some background.

For this program,
```
package foo
import "syscall"
var flags uint32 = syscall.AF_INET
```
the go/types package will (correctly) say that the *types.Scope of the `flags` var is
the package scope rather than the file scope. That, however, makes it harder to evaluate
the RHS of the assignment outside of the assignment context, since the
"syscall" name is not defined at package scope.

Please provide an easy way to get from the *ast.ValueSpec (or any ast.Expr, or a
token.Pos) to the narrowest scope that encompasses its definition.