Cross-module support

This patch updates the controller-gen package loader to support
nested Go modules. If the syntax for the paths= argument ends with
four dot-characters "/....", then this now indicates to the loader
that any nested Go modules in that path should also be considered
when loading packages.

pkg/loader/loader.go

@@ -25,6 +25,9 @@ import (
 	"go/types"
 	"io/ioutil"
 	"os"
+	"path"
+	"path/filepath"
+	"strings"
 	"sync"
 
 	"golang.org/x/tools/go/packages"
@@ -341,13 +344,70 @@ func LoadRootsWithConfig(cfg *packages.Config, roots ...string) ([]*Package, err
 	// put our build flags first so that callers can override them
 	l.cfg.BuildFlags = append([]string{"-tags", "ignore_autogenerated"}, l.cfg.BuildFlags...)
 
-	rawPkgs, err := packages.Load(l.cfg, roots...)
-	if err != nil {
+	// check each root to see if it should be expanded to include nested modules
+	var goModDirs []string
+	findGoModules := func(p string, d os.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if !d.IsDir() && path.Base(p) == "go.mod" {
+			absPath, err := filepath.Abs(p)
+			if err != nil {
+				return err
+			}
+			goModDirs = append(goModDirs, path.Join(path.Dir(absPath), "..."))
+		}
+		return nil
+	}
+	for i, r := range roots {
+		// skip roots that do not end with /....
+		if !strings.HasSuffix(r, "/....") {
+			continue
+		}
+		// update the root to no longer descend into nested modules
+		roots[i] = r[:len(r)-1]
+		// add any nested modules to the list of roots
+		if err := filepath.WalkDir(r[:len(r)-4], findGoModules); err != nil {
+			return nil, err
+		}
+	}
+
+	// loadRoots parses the provided file paths and returns their load packages
+	loadRoots := func(roots ...string) error {
+		rawPkgs, err := packages.Load(l.cfg, roots...)
+		if err != nil {
+			return err
+		}
+		for _, rawPkg := range rawPkgs {
+			l.Roots = append(l.Roots, l.packageFor(rawPkg))
+		}
+		return nil
+	}
+
+	// load the packages from the main module
+	if err := loadRoots(roots...); err != nil {
 		return nil, err
 	}
 
-	for _, rawPkg := range rawPkgs {
-		l.Roots = append(l.Roots, l.packageFor(rawPkg))
+	if len(goModDirs) > 0 {
+		// ensure the working directory is updated back to its original location
+		workingDir, err := os.Getwd()
+		if err != nil {
+			return nil, err
+		}
+		defer os.Chdir(workingDir)
+
+		// load the packages from the nested modules
+		for _, p := range goModDirs {
+			// change the working directory to the root of the nested module
+			if err := os.Chdir(path.Dir(p)); err != nil {
+				return nil, err
+			}
+			// load the packages from the nested module
+			if err := loadRoots("./..."); err != nil {
+				return nil, err
+			}
+		}
 	}
 
 	return l.Roots, nil