go/types: type check is failed with custom importer (golang.org/x/tools/go/packages)

main.go

package main

import (
	"go/ast"
	"go/parser"
	"go/token"
	"go/types"
	"io/ioutil"

	"golang.org/x/tools/go/packages"
)

func main() {
	src, err := ioutil.ReadFile("project/main.go")
	if err != nil {
		panic(err)
	}

	fset := token.NewFileSet()
	f, err := parser.ParseFile(fset, "main.go", string(src), parser.Mode(0))
	if err != nil {
		panic(err)
	}

	conf := types.Config{Importer: &Importer{}}
	if _, err = conf.Check("main", fset, []*ast.File{f}, nil); err != nil {
		panic(err)
	}
}

type Importer struct{}

func (p *Importer) Import(path string) (*types.Package, error) {
	return p.ImportFrom(path, "", 0)
}

func (i *Importer) ImportFrom(path, dir string, mode types.ImportMode) (*types.Package, error) {
	cfg := &packages.Config{
		Mode: packages.LoadTypes,
		Dir:  dir,
	}
	pkgs, err := packages.Load(cfg, path)
	if err != nil {
		return nil, err
	}

	if len(pkgs) > 0 {
		return pkgs[0].Types, nil
	}
	return nil, err
}

project_main.go

package main

import (
	"fmt"
	"os"
	"path/filepath"
)

func main() {
	filepath.Walk(".", walker)
}

func walker(path string, info os.FileInfo, err error) error {
	fmt.Print(path)
	return nil
}