cmd/go/internal/modload: remove ImportMap and PackageDir

These two functions together duplicated much of the functionality of
modload.Lookup. Use that instead in modcmd.vendorPkg, and reduce the
modload surface area.

Updates #42504
Updates #40775
For #26904

Change-Id: Ib8aaac495d090178dd56971aef9e5aa44ffa818b
Reviewed-on: https://go-review.googlesource.com/c/go/+/332571
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

src/cmd/go/internal/modcmd/vendor.go

@@ -222,20 +222,25 @@ func moduleLine(m, r module.Version) string {
 }
 
 func vendorPkg(vdir, pkg string) {
-	// TODO(#42504): Instead of calling modload.ImportMap then build.ImportDir,
-	// just call load.PackagesAndErrors. To do that, we need to add a good way
-	// to ignore build constraints.
-	realPath := modload.ImportMap(pkg)
-	if realPath != pkg && modload.ImportMap(realPath) != "" {
+	src, realPath, _ := modload.Lookup("", false, pkg)
+	if src == "" {
+		base.Errorf("internal error: no pkg for %s\n", pkg)
+		return
+	}
+	if realPath != pkg {
+		// TODO(#26904): Revisit whether this behavior still makes sense.
+		// This should actually be impossible today, because the import map is the
+		// identity function for packages outside of the standard library.
+		//
+		// Part of the purpose of the vendor directory is to allow the packages in
+		// the module to continue to build in GOPATH mode, and GOPATH-mode users
+		// won't know about replacement aliasing. How important is it to maintain
+		// compatibility?
 		fmt.Fprintf(os.Stderr, "warning: %s imported as both %s and %s; making two copies.\n", realPath, realPath, pkg)
 	}
 
 	copiedFiles := make(map[string]bool)
 	dst := filepath.Join(vdir, pkg)
-	src := modload.PackageDir(realPath)
-	if src == "" {
-		fmt.Fprintf(os.Stderr, "internal error: no pkg for %s -> %s\n", pkg, realPath)
-	}
 	copyDir(dst, src, matchPotentialSourceFile, copiedFiles)
 	if m := modload.PackageModule(realPath); m.Path != "" {
 		copyMetadata(m.Path, realPath, dst, src, copiedFiles)

src/cmd/go/internal/modload/load.go

@@ -767,28 +767,6 @@ func (mms *MainModuleSet) DirImportPath(ctx context.Context, dir string) (path s
 	return ".", module.Version{}
 }
 
-// ImportMap returns the actual package import path
-// for an import path found in source code.
-// If the given import path does not appear in the source code
-// for the packages that have been loaded, ImportMap returns the empty string.
-func ImportMap(path string) string {
-	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
-	if !ok {
-		return ""
-	}
-	return pkg.path
-}
-
-// PackageDir returns the directory containing the source code
-// for the package named by the import path.
-func PackageDir(path string) string {
-	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
-	if !ok {
-		return ""
-	}
-	return pkg.dir
-}
-
 // PackageModule returns the module providing the package named by the import path.
 func PackageModule(path string) module.Version {
 	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)