go.tools/imports: recognize exports in cgo files

Fixes golang/go#8815.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/143660044

imports/fix.go

@@ -283,7 +283,8 @@ func loadExportsGoPath(dir string) map[string]bool {
 		return nil
 	}
 	fset := token.NewFileSet()
-	for _, file := range buildPkg.GoFiles {
+	for _, files := range [...][]string{buildPkg.GoFiles, buildPkg.CgoFiles} {
+		for _, file := range files {
 			f, err := parser.ParseFile(fset, filepath.Join(dir, file), nil, 0)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "could not parse %q: %v\n", file, err)
@@ -295,6 +296,7 @@ func loadExportsGoPath(dir string) map[string]bool {
 				}
 			}
 		}
+	}
 	return exports
 }