cmd/compile/internal/noder: remove unified IR assumptions on LocalPkg.Path == ""

Replace `pkg.Path == ""` check with `pkg == types.LocalPkg`. This is a
prep refactoring for CL 393715, which will properly initialize
types.LocalPkg.

Updates #51734.

Change-Id: I7a5428ef1f422de396762b6bc6d323992834b27c
Reviewed-on: https://go-review.googlesource.com/c/go/+/406056
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/noder/linker.go

@@ -153,7 +153,7 @@ func (l *linker) relocObj(pr *pkgReader, idx int) int {
 	l.relocCommon(pr, &wdict, pkgbits.RelocObjDict, idx)
 
 	var obj *ir.Name
-	if path == "" {
+	if sym.Pkg == types.LocalPkg {
 		var ok bool
 		obj, ok = sym.Def.(*ir.Name)
 

src/cmd/compile/internal/noder/reader.go

@@ -295,13 +295,13 @@ func (r *reader) doPkg() *types.Pkg {
 	if pkg.Name == "" {
 		pkg.Name = name
 	} else {
-		assert(pkg.Name == name)
+		base.Assertf(pkg.Name == name, "package %q has name %q, but want %q", pkg.Path, pkg.Name, name)
 	}
 
 	if pkg.Height == 0 {
 		pkg.Height = height
 	} else {
-		assert(pkg.Height == height)
+		base.Assertf(pkg.Height == height, "package %q has height %v, but want %v", pkg.Path, pkg.Height, height)
 	}
 
 	return pkg

src/cmd/compile/internal/noder/unified.go

@@ -232,7 +232,7 @@ func readPackage(pr *pkgReader, importpkg *types.Pkg) {
 	r := pr.newReader(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
 
 	pkg := r.pkg()
-	assert(pkg == importpkg)
+	base.Assertf(pkg == importpkg, "have package %q (%p), want package %q (%p)", pkg.Path, pkg, importpkg.Path, importpkg)
 
 	if r.Bool() {
 		sym := pkg.Lookup(".inittask")