cmd/go/internal/vet: print line numbers appropriately on list errors

Fixes golang#36173

For reasons that are unclear to me, this commit:
golang@f1d5ce0
introduces a TestPackagesFor function that strips line numbers from error
messages. This commit introduces a new version of that function for 'go vet'
that always keeps the line numbers.

src/cmd/go/internal/get/get.go

@@ -274,7 +274,7 @@ func download(arg string, parent *load.Package, stk *load.ImportStack, mode int)
 		stk.Push(arg)
 		err := downloadPackage(p)
 		if err != nil {
-			base.Errorf("%s", &load.PackageError{ImportStack: stk.Copy(), Err: err})
+			base.Errorf("%s", load.NewPackageError(stk, err))
 			stk.Pop()
 			return
 		}
@@ -353,10 +353,8 @@ func download(arg string, parent *load.Package, stk *load.ImportStack, mode int)
 			}
 			if j, ok := load.FindVendor(orig); ok {
 				stk.Push(path)
-				err := &load.PackageError{
-					ImportStack: stk.Copy(),
-					Err:         load.ImportErrorf(path, "%s must be imported as %s", path, path[j+len("vendor/"):]),
-				}
+				err := load.NewPackageError(stk,
+					load.ImportErrorf(path, "%s must be imported as %s", path, path[j+len("vendor/"):]))
 				stk.Pop()
 				base.Errorf("%s", err)
 				continue