Fixes rjeczalik#35: Slow to generate interfaces

Migrate from using the older `loader` package in golang
tools to the new `packages` package in golang tools.  The
previous `loader` package does not support modules and
after switching from `loader` to `packages` it modified
an execution that took over 10s to under 1s.

interface.go

@@ -2,12 +2,11 @@ package interfaces
 
 import (
 	"errors"
-	"fmt"
 	"go/types"
 	"sort"
 	"unicode"
 
-	"golang.org/x/tools/go/loader"
+	"golang.org/x/tools/go/packages"
 )
 
 // Interface represents a typed interface.
@@ -65,43 +64,35 @@ func (i Interface) Deps() []string {
 }
 
 func buildInterface(opts *Options) (Interface, error) {
-	cfg := &loader.Config{
-		AllowErrors:         true,
-		Build:               opts.context(),
-		ImportPkgs:          map[string]bool{opts.Query.Package: true},
-		TypeCheckFuncBodies: func(string) bool { return false },
-	}
-	cfg.ImportWithTests(opts.Query.Package)
-	prog, err := cfg.Load()
+	var err error
+
+	// If a requested type is defined in an external test package try to
+	// build the interface using it before returning an error.  Therefore,
+	// set the `Tests` flag to true so that any test packages are also
+	// loaded and checked below.
+	cfg := &packages.Config{
+		Mode:  packages.NeedTypes | packages.NeedTypesInfo | packages.NeedImports | packages.NeedDeps,
+		Tests: true,
+	}
+
+	pkgs, err := packages.Load(cfg, opts.Query.Package)
 	if err != nil {
 		return nil, err
 	}
-	pkg, ok := prog.Imported[opts.Query.Package]
-	if !ok {
-		return nil, fmt.Errorf("parsing successful, but package %q not found",
-			opts.Query.Package)
-	}
-	i, err := buildInterfaceForPkg(pkg, opts)
-	if err == nil {
-		return i, nil
-	}
-	// If a requested type is defined in an external test package try to
-	// build the interface using it before returning an error.
-	queryCopy := *opts.Query
-	queryCopy.Package += "_test"
-	optsCopy := *opts
-	optsCopy.Query = &queryCopy
-	for _, pkg := range prog.Created {
-		if pkg.Pkg.Path() == optsCopy.Query.Package {
-			return buildInterfaceForPkg(pkg, &optsCopy)
+
+	for _, pkg := range pkgs {
+		opts.Query.Package = pkg.Types.Path()
+		i, err := buildInterfaceForPkg(pkg, opts)
+		if err == nil {
+			return i, nil
 		}
 	}
 	return nil, err
 }
 
-func buildInterfaceForPkg(pkg *loader.PackageInfo, opts *Options) (Interface, error) {
+func buildInterfaceForPkg(pkg *packages.Package, opts *Options) (Interface, error) {
 	var typ *types.Named
-	for _, obj := range pkg.Defs {
+	for _, obj := range pkg.TypesInfo.Defs {
 		if obj == nil {
 			continue
 		}