cmd/compile: fix lazy loading in reader2

Calling reader2.obj fully loads the referenced object, which is
necessary in general; but for reading the package index, we just need
to setup the name->index mapping. This CL adds this, so that lazy
loading works as intended.

Change-Id: Ie51d59e2247d99b46f9dc69fba7ce89e2584b7c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/348011


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/noder/reader2.go

@@ -43,7 +43,12 @@ func readPackage2(check *types2.Checker, imports map[string]*types2.Package, inp
 	r.bool() // has init
 
 	for i, n := 0, r.len(); i < n; i++ {
-		r.obj()
+		// As if r.obj(), but avoiding the Scope.Lookup call,
+		// to avoid eager loading of imports.
+		r.sync(syncObject)
+		assert(!r.bool())
+		r.p.objIdx(r.reloc(relocObj))
+		assert(r.len() == 0)
 	}
 
 	r.sync(syncEOF)