gogetdoc seems not to work when a go file has import "C"

[leixure]

foo.h

#ifndef __FOO_H__
#define __FOO_H__

inline void foo(void)
{
}

#endif//__FOO_H__

foo.go

// Package foo is just any package
package foo

/*
#include "foo.h"
*/
import "C"

// Foo calls the foo function in C
func Foo() {
    C.foo()
}

bar.go

package foo

// Bar calls Foo
func Bar() {
    Foo()
}

gogetdoc for foo.Foo and foo.Bar and see the former fails and the latter succeeds

$ gogetdoc -pos "foo.go:#125"
xxx/bar.go:5:2: undeclared name: Foo
gogetdoc: couldn't find foo.go in program
$ gogetdoc -pos "bar.go:#37"
import "xxx/foo"

func Bar()

Bar calls Foo

Now change bar.go to import "C"

package foo

/*
void bar()
{
}
*/
import "C"

// Bar calls Foo
func Bar() {
    C.bar()
}

then run gogetdoc for foo.Bar

 $ gogetdoc -pos "bar.go:#70"
gogetdoc: couldn't find bar.go in program

[zmb3]

Cgo is not currently supported, so closing this as a duplicate of #33 and #21.

I didn't find the performance with Cgo enabled to be acceptable, but I'm hoping with the advent of go/packages (see #38) we'll be able to resolve that.